QinQ involves use multiple VLAN tags in an Ethernet header so that one VLAN ID can carry another 4096 VLAN IDs in a second tag. This makes a simple and useful tunnelling strategy.
The first/inner tag is the one set by the customer, and the second/outer tag would be set by the network. It’s common in the Service Provider industry 1 to refer to these as Customer VLAN and Service VLAN.
