Basically after level 30 all Disciples of war or magic gain a "job" those o so familiar names from FF's past, to unlock each job you also need to have the requisite sub class leveled to 15.
This guy covers all the combinations required to unlock each job.
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Additionally note as of now each disciple of war and magic have 1 job they advanced to but it has been anounced in the future classes may have additional jobs they can unlock.
Bit of conjecture here however with the addition of arcanist there were also be SMN and SCH all 3 which are book wielding classes and have released a teaser showing all 3 classes to have some form of summoning anyways likely arcanist will be the core base class for both jobs.
That said would be interesting RDM requires Gladiator 30 (more duelist style) and thaumaturge 15 would focus more on enhancing sword with elemental magic rune fencer style <.<
Dark knight requires marauder 30 and Gladiator 15.
Duelist (pistol wielder or perhaps corsair?) requires Arc 30 and... Gladiator 15 (or thaumaturge 15 if more corsair style?)
heh anyways just having some fun speculating at future classes x.x