cmSysShelfCount is a read-only integer between 1 and 4 that reports how many physical shelves are currently reporting into this multi-shelf DSL system. An administrator monitors it to detect when an expansion shelf drops offline, since the count would decrease from its expected value. For example, if a four-shelf deployment suddenly reports cmSysShelfCount as 3, the operator knows one shelf has lost communication with the system controller and should check its uplink or power.