Use of Magic
Every caster belongs to a school of Magic (e.g. Thinkamancy or Croakamancy) and can cast spells from that school. Casting spells uses Caster's "juice" so there is a limit on the amount of magic a Caster can perform during one turn .
Casters are also able to cast spells (in form of scrolls) from other schools of Magic (apparently with varying efficiency) as well as their own school. Apparently casting spells from scrolls does not spend Caster's "juice".
However, they almost never do so since they are too rare and valuable to risk. Moreover, they do not have Leadership ability and thus give no Leadership bonus to units under their direct command. In some special cases they do give a bonus to the lead troops: known examples are Golems led by a Dirtamancer and uncroaked led by a Croakamancer).