They could certainly have 7 spots that all are "P" if they wanted, which would mean you could put any pitcher in any spot. By making some of them SP and RP spots, it makes it harder on the teams. Can't put a starter in a reliever spot and vice versa. Though some starters have "RP" status (because they started out in the bullpen or were projected to), and thus would be more flexible because you can put those starters in any spot.
Basically what your league rules are generally saying is that you can start up to 5 SPs or 5 RPs per day max, and up to 7 total pitchers.