Umm well to a extent it's like FFT. You have a main class and then at level 18 you can do the sub class quest. A sub class let's you have the abilities (minus the 2 hour) and stats of the sub class. The sub class also can onyl be half the level of the main. So let's say your a 20 warrior and subbing as a monk, your monk can onyl be level 10, if it's over you only receieve the abilites and stats of a level 10 monk but your monk levels still exsist just can't be used until you level your warrior up more. In addition only the main class gains exp.

But something I should mention, mixing magic and fighting classes isn't a good choice. You can do it, but it's generally not worth it. Mainly because a warrior's stats can't add much to a whtie mage, and a black mage's stats can't help out a monk all too well.

Umm I hope this helps explain sub classes.