Ski-Bird:
Can someone shed some light on the numbered groups?
Groups can also be useful as an organizational tool.
For instance, you can use the lettered groups for specific purposes. You might make posts that explain game rules in Group R (for "Rules") and player-owned threads for notes in Group N. You'll be able to see everything, but players will only be able to see posts in the groups you give them, and they can only post in non-Closed threads.
For instance, if you posts threads for your own notes in Group U, just don't give any players access to that group. If you make a thread for a player to use for listing spells, you might put it in Group S, then only give that player access to that group. No one else needs it, so they don't need to see it.
Any threads you make in Group 0 are visible to users who aren't players in your game. You can use this to make introductory posts (like explaining the setting, or what you want in an RTJ). Some GMs here keep the main story thread, and/or the OOC thread, in Group 0 so that outsiders can follow along.