Volunteer troops as opposed to conventional troops are composed of soldiers volunteered by provinces affiliated with entities such as clans, temples and other similar organizations.

Conventional troops are recruited from your peasant base as militia and then trained into specialized troops. Volunteer troops are offered to you as tribute per turn. They do not require payment in gold but they do require you feed them as you do normally other troops.

When you build a clan hall or similar buildings which give volunteer troops as a bonus, you will receive one of their warrior classes every few turns as tribute for allowing them to settle in your city. This warrior must be exchanged with one of your militia, so make sure you always have a handful of militia available to make the exchange at the end of each turn. At the start of the turn you can check your army list for any new volunteers. If you do not have militia available to make the exchange the volunteers will be discarded.