This allows you to change any skill set of any unit with the following exceptions:
1) You cannot give mime any skillset. You cannot add to, or replace the skills of samurai, ninja, or chemist; you can only delete.
2) Any skill belonging to a skillset with a special action menu (see here) cannot co-exist in a skillset with other types of attacks. As a logical corollary, any attempt to modify such skillsets (as long as it's not samurai, ninja, or chemist) must completely replace all the original skills and the appropriate action menus.
3) Any duplicate of the above skillsets cannot be used on player-controlled units.
4) If a special unit class shares any skillset with a generic unit class (like if you changed Orlandu to have his base skillset be knight breaks), the mastery of that skillset will depend on the mastery of the generic unit class.
5) Any non-generic skillset with the default action menu, if set as a secondary, will be learned by the player or AI regardless of JP cost (the only factor is the chance to learn).
6) You cannot put reaction abilities under the ability slot and vice-versa.