It's a forum meant to highlight d20-system games and give them more attention on RPGnet. We've traditionally underserved the d20 community; the new forum is an attempt to correct that, to bring new readers to RPGnet, and to better serve existing readers who also play d20 games.
What Post Goes Where?
The short and easy answer is that if a post derives from a d20-based game or d20-based setting, it should go in the d20 forum.
More specifically: if a post is exclusively about a d20-system game, whether it's talking about crunchy mechanics or impact on the industry, it belongs in d20. If it's about a d20-based setting, even if it's mechanics free, it belongs in d20. If it's a comparison or discussion of multiple d20-system games, it belongs in d20. If it's a post about porting a traditional setting or game system to d20, it belongs in d20.
There are some posts which are broader. These include posts that compare or discuss multiple systems, some of which are d20 and some of which aren't, and posts which talk about using traditional d20 settings with other game systems. For the comparative posts, feel free to post them in whichever forum you think will get a better response. For posts about other game systems, please post them in Open.
(And though it shouldn't need to be said: making an offhand mention of more systems just as an excuse to post in a different forum isn't any more cool here than it would be if you were using such a trick to post in RPG Open rather than Tangency. Forums live by the spirit of their rules, not the letter, and that's how we judge things at RPGnet.)
What's a d20-System Game?
It's any game clearly and directly derived from Wizards of the Coast's d20 SRDs or Dungeons & Dragons (in any edition).
This clearly includes any version of D&D, including: the original 3-book set, Basic Dungeons & Dragons, Advanced Dungeons & Dragons, or the modern Dungeons & Dragons.
It includes any games or supplements derived from versions of OD&D, BD&D, or AD&D, such as the Mayfair Role Aids products (where were unsanctioned AD&D supplements), or HackMaster (which is built on an AD&D License).
It most obviously includes any games depicting the d20 trademark and any games depicting the Dungeons & Dragons trademark, from Dungeon Crawl Classics to Kalamar.
It includes any games built on the d20 SRDs, the d20 modern SRDs, or the d20 future SRDs. Besides having the immediately familiar d20 mechanics these games will also typically have an OGL in the back of the book. It includes numerous systems in all genres, including Babylon 5, Castles & Crusades, Conan, Mutants & Masterminds, True 20, and many others. These OGL (d20) games are effectively the current generation of d20 games, brought about because of the trust collapse in the d20 trademark that occurred in 2003-2004.
It finally includes any games released by Wizards with largely similar d20 systems, which currently means Star Wars.
This list isn't meant to be exhaustive, and the basic rule is very simple: if it's D&D based, it's what we call a d20-system. We chose this most expansive definition because it's also the clearest.
