Reading the latest Dr. Deveraux article about doing world-building and the how's and why's of recruitment in a pre-modern and pre-industrial society actually went, and it's very interesting because the how of the recruitment of soldiers is pretty directly tied to the why of the recruitment.
Now it's a very broad and complex topic, but he succinctly narrows the styles of recruitment of four main ways:
The Employment Principle - "I fight because I'm paid to fight."
The Entitlement Principle - "I fight because I have personal rights/legal status to fight." (The article goes into the more complex detailing on that)
The Vocational Principle - "I fight because it's my job" - Not the same as the 'Employment Principle' because this is more focused around the 'Warrior Class' in history, either as warrior aristocracy, warrior slaves, warrior settlers to use Dr. Deveraux's examples.
The Clientage Principle - "I fight because I'm obligated to". This is the one you see most often in the Middle Ages thoroughly, and practically all across Eurasia.
All of these are mutually exclusive from the other (a warrior slave in the warrior class is not a client, because it his vocation to fight not a duty expected of him from a lord) but they were not mutually exclusive in a society and each would have filled their own distinct niches in a society.