If a guard has an Avantix in their hand, they can sell any national (regional) rover product. If they think they can't, it's either a training issue, or they haven't issued one for so long they can't remember how to do it and don't have the time or the inclination to find out.
Each regional rover product is assigned to and managed by a franchised TOC. The owning TOC is responsible for price and validity changes, including which TOCs do or do not participate. The open access operators, which came along after privatisation, were for that reason not automatically included in the rover scheme from day 1 of the privatised world. If an operator is missing from the list of valid TOCs for a rover product, either they have not been invited (or they have not asked) to join, and will receive no revenue from it.
The All Line Rover is different in that it is managed by RDG, with price and validity changes agreed by TOC representatives collectively.