opinion and advice on ansys elements and commands

sir ,i ahve some problems in use ansys element CERIG,MPC 184 and RB3 elements....what is difference in transfering loads to slave nodes using MPC184 elements and command CERIG and RBE3 ..

and also why mass21 elements are required if u defining coonection usimg CERIg and RBE3 command..

plz...advice and clarify my doubts to understand better..

thanks and regards..

-dileep