Unsigned64 is the companion bitmask to adminPropMask on the same gmeta:Class registry entry, but it marks which properties of that managed-object class the currently running firmware actually populates at runtime (its operational surface), which can be a subset of what the schema defines if a feature is disabled or the endpoint hasn't reported in yet. Tooling compares operPropMask against adminPropMask to know whether a given property is safe to read right now versus merely defined in the schema. For example, a fabric-interconnect class's operPropMask bit for a firmware-version property might be clear until that FI has finished its post-boot inventory handshake.