As soon as they are ready (usually a few years after joining their faction, though some are later), Anyeli must join a caste if they want to learn powers and abilities specific to their role in society.
Choosing not to join a caste is not as damaging as not joining a faction, but the benefits of specialising (both in terms of power and the Anyelos's standing in the Empire?) are usually worth doing so.
Anyeli must have at least 5 skill points spent in a class-based skill (such as Aura Form or Aura Shield? for Apali, or Shadow Cloak? or Shadow Form? for Skotadi) in order to choose their caste specialty.