You dont have to take a specific subjob. Abilities from
any job can be combined with your main (what you are currently playing as) job, as long as you have enough action points, and it wont really be a detriment to your overall stats. Of course, some skill combinations will make more sense than others.
Also, it looks like there is equipment that everyone can wear, equipment thats only for certain discipline sections (war,magic, hand, etc.), or equipment that is specific to a certain job.
http://twitpic.com/27unj5
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