i've heard someone or other mention about it from time to time on forums but have no clue what it is
can anyone explain?

this got burried already?

there are 3 divisions of about 20 Players, 20A, 20B and 20C. it's an invitional currently runned from JSA i think. some relegate or promote every season. i think the winner from 20C Comes to 20A the following season. No idea how many promote from B. I'm not so informed about this, but i think with this shorth description you get at least a few informations

https://www.warlight.net/Forum/86250-20-proving-ground-season-6