This read-only port list reports, only while double VLAN is disabled, which ports are instead connected to provider or customer VLANs using standard 802.1q tagging; the list is empty whenever double VLAN is enabled. An admin reads it while double VLAN is off to see which ports are still associated with the older single-tag 802.1q VLAN configuration, useful during a migration to QinQ. For example, an admin transitioning a switch to double-VLAN mode would check this list beforehand to understand the standard 802.1q port assignments already in place.