What all does magic allow one to do in the Guilty Gear world? Outside of the obvious such as fire control and whatnot, does it allow for other feats such as healing? Or the more "far fetched" abilities like making food appear out of nowhere?
Well, Magic is...in so many words an 'inexplicably limitless' (at least in terms of current knowledge of it) energy source.
Energy can take many forms and be used for a variety of things.
Obviously the most basic of these is to serve as a "power source".
So you can bet that Magic in the height of it's development and discovery was instantly substituted for Fossil Fuels (Automobiles/Planes), Electricity (Appliances/Public Works), Solar Power, Nuclear Power, Wind Power...and so on.
Maybe it did have it's objectionist groups who questioned the reliability of Magic (as is currently being done with mixed Hybrid Fuels in these times), but it didn't take long before those groups were likely silenced by the progression and development of Magic Particle Physics.
The basic structure of Magic casting...or the "Magic Arts" (Houjutsu/法術) requires "Mana" (Houriki/法力).
This is very similar to "MP" that Mages/Sorcerer-types use in Role-playing games.
"Houriki" in its base definition means "Magic Ki" or Magic Energy.
In case you didn't know, "Ki" flows throughout all of the world and is a primary component of life. You might say "Ki" is the founding element.
Of the elements we have Ki (of course), Lightning, Wind, Water, and Fire.
This is known as the "Five Element Configuration", or "Five Context".
According to this basic belief, all energy takes these 5 forms, Ki being the most internalized and basic.
"Houriki" or "Mana" is a little different from Ki energy in and of itself because you are utilizing your own energy to borrow energy from another source.
In other words, you are using your own energy to draw "energy" from the Backyard (バックヤード).
Obviously, using the human body as a conductor to produce Magic has its limits, so they turned to using both Machines and other bio-technologies (read: Gears) as a means of tapping this seemingly unlimited resource.
Of course, there's more to it than that...
When casting specific Elemental spells, one must be naturally "attuned" to that element...so, for example Sin, who is naturally "attuned" to the Lightning Element, cannot cast Fire spells very well...so logically he can't borrow Sol's Firesealed Sword.
The Magic Arts also have another requirement besides "Mana" and being "Attuned"...in order to be controlled properly, they adhere to something known as the "12 Steps".
This of course is a format and technique that was developed over time throughout the history of Magic's use.
The 12 Steps are a codex of patterns and methods that were regulated to cast specific types of spells...a "Chromatic Scale" if you will.
Much like how Music is composed, Magic must be well-composed in order for it to have logical structure and be properly utilized without raging out of control.
It's said (and rightfully so) that Lightning and Fire are the most difficult elements to control, and potentially the most unpredictable and dangerous.
HOWEVER, spell-casting of that nature is not the only means or method of casting Magic.
First, as you pointed out, Healing Magic does exist. The exact structure of Healing Magic is most likely linked to basic Ki as an internal self-enhancing element. Speeding up a body's natural healing capabilities for example.
Second, you have what are known as "Magic Seals". Sometimes they are called Barriers or Dimensional Seals...these can be laid out using any number of ritualistic setups...but in the case of Guilty Gear it is more common for "Blood Seals" to exist...so in other words, blood is used as a conduit for the barriers (or in rare cases, Dimensional Gates). These barriers are sometimes known as a 4th or Quatrad (for their 12 Step composition), also called Nth Mapping, Space Curve, or Ouroboros Loop.
Barriers were also used during the Crusades by the "Houshien" Squad or "Magic Support" squads. Aside from Healing Magic, this group was the first and last members of the Sacred Order to leave the battlefield and it was their job to protect the other squads and support them as much as possible by deflecting Magic and reducing the damage.
Third, there are some spells that exist outside the normal range of the "12 Steps" standard. More commonly known are the 6 Forbidden Dark Bondage Laws (Hi-Deigokutsuipo/卑・泥獄墜法) which sacrifice one of a human being's natural sensory abilities in exchange for a Forbidden Beast (Kinjuu/禁獣) Parasite. Aside from those spells...there is a less common method known as "Soul Sinker", that splits a living being's 'soul' into an inter-spacial object that can summon 'servants' for it's 'Master' from a structure known as a "Master Ghost".
Lastly, and perhaps more obscure, is Alchemy. Though it's not completely understood in the realm of Guilty Gear, it is clear it is a very powerful technique. It's unclear how Alchemy relates to the Magic Arts, or even IF they are in fact related at all...but it has produced War Relics and Homunculi in the past, so it should be understood as a valid power in and of itself.
Even less understood, however, is the nature of the Five Context as it relates to the Backyard.
Currently there is only a slim group of individuals who fully understand the nature and structure of Gears (Biological Weapons/生体兵器) and Youkai (妖怪) and how they relate to the Backyard. But because these species (either manufactured or born) are related to the Five Context of Elements, it means that they are (as stated earlier) "attuned" to certain Magic structures.
Being "attuned" to Magic, or "tied" to a certain element can also have an impact on how powerful a being can be.
For example, because Sol is "well-attuned" to "Fire Magic" he can cast extremely destructive spells (not simply because he wields the Fireseal, but also because his Gear body is linked to the Fire Element).
So, logically, you can say that there are "High-Class" elemental beings within a caste of a certain element...in folklore and mythology, we call these beings "Elemental Deities".
It's speculated that these "Deities" or "Sealed Gods" are the power-source for the 8 Sacred Treasures, otherwise known as the "OutRage".
According to Frederick, the reason these 8 "Sacred Treasures" (Jinki/神武) were split up in to eight fragments was because the Magical Power potential had no controllable limit.
Outside of the eight OutRage, there are Gears (Dizzy) and War Relics (such as Sealed Dragon Arm Fenrir and its host Tyr) that are linked to these powerful "Deities".
Then of course, there is the mystery behind the "Isolated Space" structure within the Backyard known as the Cube that was created by "That Man" (The Gearmaker).
The complexity and variance of the Magic and Mana used for the OutRage and the Cube (not to mention Gears) are probably not currently fully known, and may not be known for some time to come.
On a more basic level, though...Magic is utilization of Energy Molecules (similar to basic Particle Physics). Things like Teleportation (i.e. Faust), Summoning (i.e. Testament), Sublimation (Defragmenting a physical object, i.e. Valentine), Temporal Change (i.e. I-No), and more are very possible.
The Mana requirements vary of course (and might even be off the scale!), but something as simple as making food appear MIGHT be possible, but again, as said earlier, it requires Magic Energy to cast something of that nature (which could potentially be dangerous).
As such, Magic is regulated (much like Blacktech or contraband technology), so you can't simply cast spells all willy-nilly as-you-like.
