cmBoardClusterRole is a read-only enumeration that, for SCM boards, indicates whether that SCM is a master (carrying WAN interfaces) or an expansion controller, and reads notApplicable for all other board types. An administrator uses it to confirm the WAN-facing SCM in each half of a redundant bay is correctly designated master before relying on it for uplink traffic. For example, if an SCM that should be handling WAN uplinks instead reports cmBoardClusterRole as expansion(3), the operator knows the WAN traffic path for that bay is not where expected.