Wondering about this myself.
What I understand so far is that you have six separate smaller sphere grids for each of the jobs in the game. Each character is naturally better at three out of those six jobs, which means it costs less for them to level up (or whatever they call it) the three jobs they're naturally good at.
Characters can change jobs on the fly in combat, I think. So each character has a different mix of jobs they're best at. Attribute increases are also mixed into the job sphere grids, so sometimes you have to choose between leveling up one job just so you get the permanent attribute bonuses, or leveling up a different job which you actually want to use more often in combat.
From what I've heard, I'm pretty excited about trying the system. I hope it isn't quite as easy to powergame by min/maxing as X and XII are, but I won't know that until I try it.
