It's worth noting that the TOCs are not (directly) involved in PlusBus implementation. The scheme is managed by Journey Solutions, a partnership of the big "names" in the bus industry; ATOC; the Association of Local Bus Managers (ALBUM) and the Confederation of Passenger Transport.
To set up a new PlusBus scheme, one of the operators in the given area takes lead responsibility and then gets in touch with any other operators asking them if they want to participate, forming a local scheme management committee. It's up to them to decide on pricing, whether to offer season tickets, zonal boundaries, revenue allocation, etc.
They then enter into a contract with Journey Solutions, who validate the scheme and get it added to the fares database and the PlusBus site, and include it in the next "newsletter" to TOCs. (Updates used to be published in Newsrail Express when that was a paper-based publication).
Once the scheme is confirmed, there's a standard form of Notice to Drivers which is supposed to be displayed at the relevant garages...Unfortunately that doesn't always prevent "That's a train ticket." type arguments, even though the tickets clearly say "PLUSBUS" on them, even in PTE areas. In the event of such problems, it's requested to
contact Journey Solutions so that they can investigate.