well, some users have asked about it, for some specific setups.it does the trick thank you spirit.
and yes this is quite bit confusing. is it used for multitenancy?
It could be used to use vlan tag over vxlan for example.
I have also some users needing triple tag ^_^. (qinq zone with double tag, and another tag at vm level).
or some users want to do qinq, with a vlan zone at proxmox level, and users manage the second vlan inside the vlan guest.
Allow to add an extra VLAN tag in the virtual machine or container vNIC configurations or allow the guest OS to manage the VLAN’s tag."
I'll try to make the doc more explicit too.