Well, I found it. It's called selective QinQ: switchport dot1q-tunnel service 15 match cvid 19, which adds SVLAN 15 to CVLAN 19 on that interface. This is potentially what happens on each interface at the L2S. The DSLAM only adds the CVLAN.
@kitz Okay, maybe NCAS slipped out of my mind. I'm yet to get round to understanding the DLM / RAP / OSS system fully so I haven't studied that page in depth yet. I know it's a complicated web of Openreach / BT Wholesale being customers to each other.
@carlT yes I'm talking about IPoE, which, if an ISP uses, would use a VPLS solution with DHCP address assignment as it can't use PPP authentication methods and then IPCP, because it's not using PPP at all. You don't need CVLANS/SVLANS if you're going to be using a VPDN with PPP because the authentication is done using CHAP with the user@domain and pass. I was under the impression that ADSL2+, where the user is connected to the MSAN would use this VPDN solution. But apparently, it doesn't? According to this diagram :
https://imgur.com/a/ucwu4Kf. That's very very interesting. If it were using PPPoE then why would per user SVLANS/CVLANS be needed? They can't be used for authentication and they're not needed for routing L2TP sessions. It does suggest, contra to BT SIN 472 Page 12, that IPoE is available for ISPs for their ADSL2+ customers, because that's a scenario where the CVLANs/SVLANs on the MSAN would be more relevant.
@DiggerOfHoles I've skimmed over it in the past but I couldn't immediately locate answers to some of my questions. Maybe I'm stupid.
Another really nice diagram is this:
https://imgur.com/a/WlCCDGL.
@DiggerOfHoles see, section 2.1.7, I'm not sure what it inserts in the PPPoE session setup exchange but that's interesting (I would have thought the bRAS supplies enough information such as the interface MAC address). Update:
https://community.cambiumnetworks.com/t5/ePMP-Networking/PPPoE-Intermediate-Agent/td-p/78211. But for DHCPv4, the L2S also needs to be a relay agent so it can add the SVLAN field as well, that's if the DSLAM relay agent doesn't unicast it to the ISP DHCP server and if it does then maybe it only inserts the DSLAM Interface MAC and CVLAN and the ISP RADIUS only uses that to authenticate. It may also be possible for the L2S to insert options into already unicasted DHCP packets by the first relay agent.
The fact that section 2.1.7 juxtaposes DSLAM, VDSL2 and PPPoE suggests to me that PPPoE can be used by ISPs for their FTTC customers and BT Wholesale does offer VPLS as well as VPDN services from the same bRAS but again,IPoE is a better option. In the case of PPPoE their RADIUSes wouldn't know or care about SVLANs or CVLANs because they'd be using user@domain and pass.