The Messaging system consists of one or more servers that perform specific roles. At the heart of the system is a server that performs the storage role. This server is supported by multiple servers in an application role, handling the telephony role, as long as the total number of ports within the system does not exceed 300.
A system is simply a way to describe the physical collection of servers that act together to provide functionality to a set of users. A system may span multiple locations, have multiple sites, or contain multiple clusters but has only one directory.