These determine what will appear when you click on the action tab in battle.
Most normal and special unit classes should use default.
Chemists (Skillsets 06 and A1) must have item to function properly.
Archers (Skillsets 08 and 9C) must have charge to function properly.
Geomancers (Skillset 11)
Samurai (Skillsets 13, 3B, and 3E) must have katana inventory to function properly.
Lancer (Skillsets 12 and 34) must have jump to function properly.
Ninja (Skillsets 14, 38, and 39) must have weapon inventory to function properly.
Calculator (Skillset 15) must have arithmeticks to function properly.
As a general rule, if you move any of the skills of any of the above skillsets to a new skillset and change the action menu of that new skillset to reflect their new attacks, your new unit class can only be used correctly by the AI.
Similarly, if you replace the movesets of ninja, samurai, or chemist and change their action menu to default, the new unit class will not be capable of using any skills.