In the original RM2 core rules, you cannot cast a spell above your own level, even though you learned lists in blocks (with MOST of the spells on the block being above your level). The ESF options allowed for overcasting (i.e. casting above your level) at large minuses (actual minus depending on how much overcasting you were doing -- i.e. how far above your level the spell was). But it was an option. In RMSS/FRP, this option was made part of the core rules (but the terminology carried over).
Also, something that has been glossed over so far, when you overcast (cast a spell over your level), if you fumble/fail in casting it, then you add 3x the overcasting mod to your spell failure roll (extra deadly).