For large BES installations, you can have more than one admin service running on multiple servers to assure that one, you have admin access and in HA (High availablity) taking down one server in your "pool" doesn't take away administration ablities.
BES is a enterprise class of server (Thus that part of it's name) and the newest version 5 is designed to be up and available with multiple instances of it running, the collection of all these processes on multiple servers are referred in a general sense, as the pool.
Large installations will use this design to assure that BES is available at all times, now if they could just make Microsoft Exchange that robust.......