I can answer one of these!
The M in MRP stands for Mature, which covers everything that might be covered by a mature rating on another piece of media - such as, say, an 18 rating on a movie. Not all 18-rated movies have sex in them, right? Some of them are rated 18 for having violence, or drug use, or strong and (to a kid) scary ideologies, or other extreme scenarios which still might not be suited to a younger person's viewing or (back to an RP context) a younger person's involvement.
The E in ERP stands for Erotic, which... yeah.
TL;DR: all ERP is MRP, but not all MRP is ERP.
The M in MRP stands for Mature, which covers everything that might be covered by a mature rating on another piece of media - such as, say, an 18 rating on a movie. Not all 18-rated movies have sex in them, right? Some of them are rated 18 for having violence, or drug use, or strong and (to a kid) scary ideologies, or other extreme scenarios which still might not be suited to a younger person's viewing or (back to an RP context) a younger person's involvement.
The E in ERP stands for Erotic, which... yeah.
TL;DR: all ERP is MRP, but not all MRP is ERP.