classes = your weapon of choice and over all play mechanics, each class/weapon has specific play mechanics.
pley mechanics
jobs- your actual role in battle and skills/traits that lend effectiveness to that role.
class specific skills. Usually skills that tie into creating the unique play mechanics of a class/weapon and some that only make sense with that weapon
overall skills, maybe used by any class though they may be learned in relation to certain other classes
job skills, only useable by certain jobs, which help define and optomize specific roles in a party.

job is the overall role, class is the playstyle, each job works with certain classes/weapons, some more specialized than others.

this system still allows for a custom class creation, while defining your role in a battle.