Functionally speaking, Classes have very little impact on gameplay functionality - as far as I'm aware, it only matters for specific spells that check Class level instead of entity level. Still, it's good flavour, useful in those cases, and a valuable dependency for the Action Resources Mutator, so here we are.

As shown above, you can assign multiple (sub)classes to an entity, divying up the character level for each Class, but the total value must equal 100%.

There isn't a direct association to spell lists, as a Death Cleric and a Necromancy Wizard may use the same custom spell list if desired, but you can ensure the group assigned matches the archetype of the entity by ensuring they've been given a certain number of specific spell lists. If multiple Class Groups are eligible to be assigned, one will be randomly chosen.

The Ability Overrides section is separate from the Abilities Mutator - this dictates the Abilities used for the respective checks, ensuring that attack/spell rolls are thematically aligned with the character archetype (can be found under the Stats component on the Entity in the Inspector).

[DEBUG] (S) ==== Starting mutator Classes And Subclasses (priority 7) ====
[DEBUG] (S) 1 potential class groups were identified - randomly choosing one
[DEBUG] (S) Added class Berserker at level 3
[DEBUG] (S) Added class Monk at level 2
[DEBUG] (S) Added class Life Domain at level 1
[DEBUG] (S) ==== Finished mutator Classes And Subclasses in 1ms ====