About supervisor auto-discovery
Since auto-discovery uses network broadcast packets, and these packets are not forwarded by routers, supervisors can only be discovered if they are on the same IP subnet as the host issuing the broadcast.
Qube workers that don't have the supervisor explicitly defined via the qb_supervisor value can issue a broadcast across the network and ask any supervisor that's running to respond. This is normally sufficient unless there are multiple supervisors on the network that can respond to the broadcast (on the same IP subnet as the worker). In this case, the qb_domain value provides a check for the auto-discovery mechanism: the qb_domain value is compared between the supervisor and the worker, and the supervisor only responds to requests from workers whose qb_domain value matches the supervisor's.