Complex sentence: The member, once inside the games world, has the choice to switch to different Dimensions, which act as servers, except members can Private Message each other over these types of Servers, unlike most MMO's.
Split into simpler sentences: The member, once inside the game's world, has the choice to switch to different "dimensions" (dims), which act as servers. However, members can private message (pm) each other over these types of servers, unlike most MMO's.