It's the VLAN assigned to the port configured for QinQ tunneling.
From the service provider's perspective, that's the VLAN they have assigned to that particular customer. The customer is unaware of the VLAN number and really doesn't care....because that interface just provides a 'tunnel' to the customer's other site (if the customer has one) where that that switchport will also have that same VLAN assigned to a port facing the customer's equipment.
