A computer running Microsoft Message Queue (MSMQ) Server that functions as the site controller for the first MSMQ site you create. The primary site controller (PSC) maintains a database of information concerning the computers and queues in the site. Each site usually has its own PSC, and you can install only one PSC for a given site.
For load balancing and failure recovery purposes, you can install one or more backup site controllers (BSCs) in each MSMQ site. BSCs contain a read-only copy of the PSC or primary enterprise controller (PEC) database. MSMQ sites do not require BSCs, but it is usually a good idea to install one or more of them in each MSMQ site. You must install a PSC or PEC before you can install any BSCs.