This.Offense:
You have a center, QB, and 5 skill players. You can have 2 RB, 1 TE, 2 WR...or 1 RB, 1 TE, 3 WR...or 0 RB, 0 TE, 5 WR...or any other combination of those 3 positions.
Defense:
You have your LBs and DBs...maybe 5 DBs and 2 LBs, or the standard 3 LBs and 4 DBs...again any combination...