This read-only integer reports the queue ID used to carry stack control information between switches in a stack, defaulting to 0 on a standalone (non-stacked) system. It matters because assigning ordinary QoS traffic to this same queue could interfere with the inter-switch control-plane traffic the stack depends on, so admins consult this value before mapping classes to queues. For example, seeing this value as 8 on a stacked switch would warn an admin not to also point a bulk-data class-map at queue 8.