SCR means that if you have 5 sets configured with the same SCR key, when the number is called all 5 will ring, and whoever picks up takes the call. If another call comes in, the caller will get busy even though 4 of the sets are actually idle. So SCR Single Call Ringing means One call at a time even though there may be multiple appearences.
MCR means that if you have 5 sets configured with the same MCR key, when the number is called all 5 will ring, and whoever picks up takes the call. If another call comes in, the caller will ring the other 4 sets and so on until all appearences are busy. So MCR Multiple Call Ringing means Multiple calls at a time can be taken up to the number of appearences.
SCN and MCN are Non Ringing versions of SCR and MCR.