Just speculation on my part, but I think some jobs will be cross-compatible with more than one class.

For example, Warrior could be a job option if you are a Gladiator, Marauder, or Lancer, you would become a Warrior specializing in that weapon. Say you want to be a Ranger, you can choose to be one through archer (a rng specializing in a bow) or musketeer (a rng specializing in a gun)

likewise, A gla with a shield, and cure/protect magic would have the choice to become a Paladin, a gla or mrd with absorb magic could become a Dark Knight, and a gla with healing/debuff/elemental magic would become a Red Mage.

In short, the class is defined by what weapon you have equipped, while the job is defined by or limits what skills you have available to you in the armoury system.