If I'm not mistaken, the terms/bounds are so ancient that the reasoning for them is a little mysterious, maybe they were assigned some deeper cosmological meaning, but we can't know. They're just used as a type of essential dignity, but it's a minor dignity. For example, Saturn has even more dignity in Aquarius, i.e. more management over its own topics, when it's in the last 5 degrees of that sign in a day chart, because that's when Saturn is also in its own triplicity and terms/bounds, and there's almost no other planet's interests involved. If any other planet or point in the chart is in a Saturn dignity, that means Saturn has influence over how that planet or point behaves, but the influence is even stronger if Saturn itself is in its own dignity, because then Saturn won't have its own behavior modulated by other influences. In a practical way it mostly means there's less interference and you don't have to think harder on symbolic combinations. If Saturn is at 10° Scorpio, for example, to nail down the interpretation of the topics ruled by Saturn you have to think about how Saturn behaves in a Mars environment that has just a little bit of a venusian flair. But then, if Mars is in Capricorn they can exchange and have what we call a reception (in this case mutual), giving Saturn a little bit of dignity, being not as subjugated to its host's interests.

Basically it's another type of dignity you can use to make the interpretation more meaningful.

Like all dignities, the terms are used in calculations to determine things like the Almuten, Lord of the Nativity etc. And then also in a predictive technique we call Primary Direction with Distribution of the bounds. Firmicus Maternus also has a whole chapter dedicated to the bounds of the ascendant degree in the Liber Quintus of his Mathesis, saying that they're important to interpret a fate and character. But be careful about how you read Firmicus, his work is not to be read like an astrology app's report, it's a didactic book on astrological interpretation. He says shit like "if the ascendant is in the terms of Saturn the native is a mean person who will kill their siblings", which is a great symbol, a great analogy, but it's not supposed to be taken literally. If you're going to read Firmicus like it's a report think of what he says as a symbol, an image, like a tarot card maybe, Idk.

Now here are three more references on the terms:

Claudius Ptolemy (2nd century Alexandria): Tetrabiblos, book 1, chapter 23. The disposition of the terms. He's a little arrogant trying to put fault in the egyptians without knowing why they distributed the bounds the way they did, so I don't like it, but do read it, it's important.

Vettius Valens (2nd century): Anthologies, chapter 3. Valens gives interpretations for each of the 50 bounds around the zodiac, which I assume is to be read of the ascendant's terms. I'm not sure which divisions he's using, if it's egyptian or ptolmaic, so it's better to check that. It's a little bit silly in my opinion, especially when he gets specific, but interesting.

Guido Bonatti (13th century Italy): Liber Astronomiae Part II (Zoller's translation). He gives a method for interpreting dignities in length from chapter XI through XIX. I should also note that Chapter XIX "Concerning the fortitudes of any planets in each of his dignities" is an interesting one, because he explains an analogy for the dignities that gets used a lot by traditional astrologers today. I find that analogy less helpful and appealing now, but you could be interested in knowing it:

The terms or bounds devide each of the signs unequally by 5. Each term has its ruler and only the Sun and the Moon dont have any bounds/terms. They're like a little tiny space in that sign that gives a little bit of dignity to a specific planet that's not necessarily the ruler of the sign. If you have Jupiter in fall in Capricorn then it's not well placed and its topics are going to suffer, but if it's in its own terms/bounds of Capricorn, then it can do something. They're very ancient and we dont know the rationale behind them, we dont know how they got calculated, why they're like how they are, or who decided the divisions. But they're used much more by horary astrologers and it works like a clock anyway. There's no horary without them.

You can mark to see them on astro.com in the extended chart selection. I use the egyptian terms because that's the most used, especially by William Lilly, who's the biggest name in horary astrology. I recomend reading Lilly's Christian Astrology for anyone interested in astrology, even if theyre not interested in horary. Ptolemy made his own thing and I dont think anyone uses his division. And the chaldean terms, I dont know about them.

The only astrologer I've read that talks about the ascendant's terms is Firmicus Maternus, from the 4th century AD. He's a dramatic guy and most people get scared of reading his delineations because everything is described very tragically. But I think you still gain something from going to his book Mathesis, on book 5 (Liber Quintus), II - The Ascendant.

Firmicus Maternus is a bit of a curiosity. Having the distinction of being the author of the most extensive treatise on astrology to survive the ancient world, the Mathesis (composed ca. 334-337), he is also the author of the Christian apologetic work, De errore profanarum religionum (composed ca. 342-346), which is one [of] the most virulent attacks of paganism in the ancient world, advocating the total destruction of pagan cult and the banning of sacrifices by force. Indeed, Firmicus has a tarnished reputation. Firmicus is the ‘poster child of Christian intolerance’. But it is not just his intolerance that has helped to tarnish it. He is also considered as a completely mediocre, second-rate writer. Turcan calls Firmicus ‘rhéteur peureux’ and De errore simply as overcompensation: a poor attempt to distance himself from his astrological work during the political rise of Christianity. Franz Cumont does not think too highly of Firmicus either, calling his Mathesis not only ‘narrow and pedantic’, but also blatantly a ‘bad treatise on astrology’. Firmicus’ conversion to Christianity is rarely taken seriously either. Hal Drake calls De errore simply ‘angling for the job’ with Christian emperors; or, as he later suggests, echoing Turcan, it was written to convince his priest that he had genuinely converted, though he questions Firmicus’ sincerity. Garth Fowden is equally dismissive, stating, ‘for all we know, he may have been a lukewarm polytheist and a Christian of convenience.’ Statements like this have relegated Firmicus Maternus to just a few pages, paragraphs, sentences, and, in some instances, complete absence in works on the history of Christianity in the fourth century. Other than what it has to say about mystery religions, Firmicus’ De errore, with a few important exceptions, is all but passed over by scholars. Ancient writers essentially ignored him too. Out of all the Latin patristic writers, only Ambrosiaster seems to have paid any attention to his apologetic work. But modern judgments and ancient neglect detract from the appreciation of Firmicus’ great value as a source of many of the complex transformations taking place in late antiquity. Certainly, Firmicus is not the caliber theologian as Jerome, Ambrose, or Augustine. But neither is Arnobius or Ambrosiaster, and both those fourth century writers have witnessed a steady increase in scholarly attention. De errore is certainly a theological work of merit, and deserves to be examined as such. The present work treats Firmicus seriously as a theologian and source of understanding conversion in the fourth century. By looking at how Firmicus understood daemons in the Mathesis and De errore, I suggest that his fervent, anti-pagan rhetoric arose, not only due to political motivations, or ‘angling for the job’, but also to Firmicus’ expressed concern about the power of evil daemons and the power Christians claimed to hold over them. The Christian co-emperors Constans and Constantius II (340-350), who are addressed in De errore, also have the power to rid the world of the Devil and his daemons en masse, Firmicus insists, through the destruction of temples, statues, and the eradication of paganism by force. [...] Whether or not Firmicus influenced, or was influenced by[,] the ban against pagan sacrifice in 341, he certainly understood greater rewards would follow the final triumph of Christianity than his own political career - it would help, he believed, to usher the final destruction of evil in the world and victory of Christ over the devil.

The Moral Orientation of Worldly Spirits

The word ‘daemon’ occurs frequently in the Mathesis, both as good and evil powers. For instance, good daemons are mentioned ten times in this work, though these daemons are primarily associated with astrological houses, and do not manifest as independent beings. Evil daemons, on the other hand, are much more common in this work. Evil daemons are mentioned seven times, cacodaemons eight; violent daemons, depraved (pravi) daemons, malignant spirits[,] and malignant daemons all mentioned once each. Thus, contrasted with good daemons, evil ones are named nineteen times in the Mathesis, almost three times as many {sic?} as good daemons. This may make sense in an astrological work, which by its nature is written to assist the individual avoid harm in his or her day-to-day lives. Furthermore, there are four instances when daemon is used without an adjective identifying them as good or evil. But, when examining the context of the use of the term, the implication is that they are all evil: two are poised to attack the innocent, others possess bodies and bring illness, and one group of daemons brings temptations. So that makes an appearance of twenty-three evil daemons, more than three times {sic???} the appearance of the good daemons. [...] As one would expect, Firmicus in De errore renders all daemons as evil, but, interestingly, mentions daemons far fewer times than in the Mathesis. Certainly, De errore is a much shorter work. Also, the angels appear even less frequently than daemons: four times total, twice mentioned in quotation of scripture, and he does not seem to be familiar with the fall of the angels’ story. His terminology for daemons in De errore, however, is more diverse than in the Mathesis. Firmicus mentions the generic daemons four times and he makes one reference to a certain ‘unclean demonic spirit’. Furthermore, evil or unclean spirits in various forms are mentioned three times. Thus evil daemons/spirits eight times in the De errore. Diablous, or the devil, however, appears some twenty times in the text. It is the devil, the master of the daemons, that is the focus of this work. The importance of understanding the name of the leader of the daemons in exorcism may explain the many mentions of the devil throughout De errore. Firmicus emphasizes that daemons are ruled by the devil. ‘The doomed serpent’ seeks to keep people from knowledge of the true Christian God. The devil ‘leads men who believe [in him] down to the abysses of hell’. Now that the cosmos is under the jurisdiction of the Christian God, and the devil’s plans have been fully uncovered, Christians have enough information about the demonic hierarchy to keep daemons under control.

Pagan versus Christian Exorcism

In the Mathesis, the power of exorcism against evil daemons takes a prominent place. In particular, exorcists are shown as experts: specialists needed to drive evil daemons from people. Indeed, the specialist exorcist is a person endowed with great powers. Firmicus understands exorcists as those ‘whose arts free men suffering from procession by daemons. They attain great profit from those skills.’ As Firmicus maintains, most powerful exorcists are ones whose astrological sign is associated with the house of Mars [... T]he heroic exorcist who has Mars on his side needs only appear and the evil daemons flee. There is also mention of exorcistic words which are used by these specialists to expel daemons. But this is the power of the exorcists alone, who are empowered by the pagan god. As we shall now see, in the De errore Firmicus stresses that all Christians are empowered to perform exorcism by use of the divine names of God and Christ, rather than what was often the case within the context above in which specialists were the only ones able to cast out evil daemons. In the Christian apologetic work De errore profanarum religionum, Firmicus states: ‘So among us the lashes of exorcistic words flagellate your gods when they start to do harm to men.’ This exultation of the power of Christian exorcism, with focus on the words rather than on specialist exorcists, permeates his Christian work. The devil and his daemons appear easily cast out uttering the name of God and his Christ. Like the Mathesis, Firmicus’ De errore emphasizes the dangers of evil daemons. But the difference is that now, specialist exorcists are unnecessary. As was the case with Tertullian, Minucius Felix, Cyprian, {and} Arnobius, all Christians, lay or clergy, have the ability to exorcise. [...] Similar to the Mathesis, exorcists in De errore are the heroic warriors against evil daemons. But now, the heroic exorcists are all Christians, including the emperors, who get their powers from Christ himself. [...] The emperors also have an important role to play in eradicating the world of daemons. What the emperors and their Christian subjects must now do is help rid the world of daemons and establish a Christian social order on earth. [...] Thus, for Firmicus, the emperors, powerful Christian exorcists, engage directly in battle with the devil alongside Christ.

Firmicus’ demonology is much indebted to contemporary Neoplatonism, especially the teaching of Porphyry. [Neoplatonic philosophy held evil daemons as an important tenant. By the third century in philosophical circles, 'the presence of evil daimons is now a central part of respectable discourse’.] Porphyry appears in both the Mathesis and De errore. He certainly speaks highly of the Neoplatonist philosopher in his astrological work, even identifying himself as his heir, calling him ‘our Porphyry’. But, conversely, Porphyry is singled out for particular attack in the De errore. He now calls Porphyry the ‘enemy of God, foe of the truth, teacher of the arts of wickedness’. In Firmicus’ Christian work, statues are the dwelling place of evil daemons and these evil beings are appeased through animal sacrifice. It is on this point that Firmicus and Porphyry would be in total agreement. However, Firmicus attempts to turn the tables on Porphyry within the context of Christian demonology. Not only are daemons nourished by the blood of the slaughtered beasts, but these daemons are under the jurisdiction of the diabolus - a point that Porphyry, as a non-Christian, did not concede. Since daemons are powerless inferior beings, Porphyry’s own views about the nature of daemons and their dependence upon the ritual acts of humans are used as a weapon against paganism in general. Referencing Porphyry’s De philosophia ex oraculis haurienda, Firmicus writes: ‘For the victims and the blood outpoured from the incessant slaughter of animals bring no effect save that the blood nourishes the substance of daemons who are procreated by the devil’. Thus Firmicus attempts to distance himself from Porphyry's demonology by recourse to Christian theology. [...] Certainly, Firmicus’ demonology has been Christianized in De errore. However, if we look closely, we can see that it is not all that different from contemporary Neoplatonism at all. Firmicus’ understanding of the Christian devil has clear resonances with Neoplatonic teaching. For example, in De abstinentia, Porphyry states that evil daemons want to be gods, and the power that rules them wants to be thought the greatest god. This ruler of the daemons shows a close affinity with Christian demonology. These evil beings for Porphyry are the ones who enjoy animal sacrifice. Also Porphyry taught that there was a master or chief daemon that wanted to imitate the Highest God. This is also the case for the Neoplatonist Iamblichus, whose concept of antitheos has clear resonances. [...] Thus, the concept of the devil as leader of the daemons in Christianity was no great leap for Firmicus, since, as a once-professed devotee of Porphyry, the groundwork had already been laid. Whether it was the devil, anti-god, or otherwise, knowledge of the leader of the daemons gave exorcists considerable power over them. [...] Daemons are ontologically subordinated due to their moral inferiority and their distance from God and his Christ. [...] Thus, in the hierarchy of Neoplatonism, Firmicus sees in Christianity a way to comprehend the whole structure of the universe within a Christological framework.