Alvin's documentation on Tenhou was made before this was implemented, so no wonder you can't find it. Gotta catch him on IRC and tell him to update that.
That "new" tab is for the Beta of "Parlor Mode" (Jansou-sen). Anyone can play. The ranking is different from the standard ranking games. It's not based on placement, but on how much you win when you win. Also, there are special rules (corresponding to mahjong parlors in Japan) like "chips" for having red dora, ippatsu, ura dora, etc, 1500 points for every honba (instead of just 300) and East/West round (i.e. East round also counts as West round). And a few other rules that will show up when you put the mouse on top of the 予約 (queue) button. If you have no idea what to do there, don't bother, just keep playing regular tenhou. There rarely is anyone to play jansou-sen with.
A better explanation on this at my post:
http://www.osamuko.com/2010/10/18/how-t ... nsou-mode/