MyLondon has also reported on this. Unusually for them, it seems a well written article
From My LondonThe dispute is over 'boundary tickets'. These are tickets which enable you to travel from the edge of a TfL» fare zone to somewhere beyond it without having to get off the train at the boundary, buy a new ticket again and continue your journey. Crucially, they avoid someone who already has a valid Travelcard for part of their route paying twice for the same journey.
The problem is, at Southeastern and SWR» , you cannot buy these 'boundary tickets' on their websites, over the phone or at ticket machines.
Instead, you have to go to a staffed ticket office counter and ask the colleague for a ticket to or from 'Boundary of Zone 6' (or whichever zone you require).
My first thought when I heard of this action a few days ago was how can these people not possibly know they can buy a ticket from Boundary zone 6 to their destination. After all I have been doing just that for approximately 30 years.
But then I wondered how I came across this knowledge. I suspect I had used my all zones annual travelcard (Gold card) to make a journey to a Central London terminus and then visited the ticket office to buy a ticket to somewhere outside London. I would have shown my Gold card to get the discounted fare and the ticket clerk must have noticed that my travelcard already covered part of the journey and must have told me that they could issue me a ticket from Boundary zone 6 instead. Armed with this new knowledge, I would then have requested future journeys from London to Network South East destinations to start from the edge of London instead.
So, I now have sympathy for those who were not aware of the boundary zone tickets.
Note: the outer zone is now zone 6 but back then it would have been either zone 5 or perhaps zone 3c.