It's even stranger for something like
Crewe to Swansea.
If your journey doesn't include ATW, you may be offered some VWC & Connections advance tickets or may not be offered any at all. If it does, it may offer some FGW/ATW & CONN ones. The description text in the route list for these will say:
Only valid on booked First Great Western and Arriva Trains Wales services and required connecting services. This fare is only valid on the train(s) specified.
but going into the code details (click on "other tickets" where it says Advance on the right, click where it says Advance further left, then click on "view the specific 'Validity Code' applicable to the journey plan that you have selected" near the bottom) gives the
text:
Valid on Arriva Trains Wales (TOC Code: ATW) and connecting services.
Seat reservations are compulsory.
This is the same as AP ATW CT & CONN, even though FGW are still in operation.