Whenever a new view is to be installed, the FLUSH protocol is run first and then the view is broadcast. The FLUSH protocol is started by sending down a FLUSH event and waiting for a FLUSH_OK event (or a timeout). Thus the FLUSH protocol has to be below the RpcGMS layer. (If a FLUSH layer was missing, RpcGMS would time out waiting for FLUSH_OK and proceed without running the FLUSH).
When a new member is to be joined, the FLUSH is sent to all current group members. The view, however, will be sent to all current members plus the new member. When a member is excluded from the group, the FLUSH is sent to all current members minus the one to be removed from the group. The view is also sent to all members minus the excluded one.