Is there really such a thing? AR7/AR8 will only do ADSL, VR9 will do ADSL and VDSL up to profile 17b (they need a third-party firmware blob for vectoring). Most devices are limited to one of the ADSL annexes (via the filters in front of the modem chipset). But I'm actually not sure if it's even possible to do VDSL but not ADSL, I would imagine that's more of a "couldn't test ADSL because don't have ADSL" scenario.
As such, it would be the choice between:
ADSL (limited to Annex-A)
ADSL (limited to Annex-B)
xDSL (ADSL limited to Annex-A)
xDSL (ADSL limited to Annex-B)
*) Like @moeller0 below says devices that can handle both ADSL-Annex-A and -B are possible, but I've also never seen one in the wild.
"xDSL" does not include "supervectoring"/profile 35b. There are currently no modems supported by OpenWrt that would do 35b. They would probably still carry filters and when used with ADSL they would still be limited to Annex A or B, so this would even prompt three additional categories instead of one, but as long as there are no supported devices anyway, that's very much bikeshedding
as far as i know, all chipsets that supports VDSL2 supports also all older standards (ADSL2+/ADSL/ADSL). There is no pure VDSL2. I might be wrong?
In this case, xDSL does not show whether the router supports VDSL2 or ADSL only.
Almost, in ADSL Annex A or B basically tell whether a modem will use frequency band compatible with POTS only (Annex A) or ISDB (Annex B), and these need to match the line-card and filters (I believe there are devices that can be switched between Annex A and B but I never saw one myself)).
In VDSL2 (and the 2 is important plain VDSL is not backward compatible with ADSL and luckily seems to be disappearing) the Annexes A, B, or C list the applicably bandplans/profiles for the U.S.A, Europe, and China (not sure where the rest of the world classifies to in this scheme), here the compatibility with POTS/ISDN depends on the profile, so modems will work all over Europe...
Fun fact Supervectoring profile 35b is specified in Annex Q of one of the VDSL2 ITU standards