So when you do G10 maps (Gazelleskins, level 70 8 man maps) you have a chance of finding a thief map. These maps have 3 doors you can pick from, and 6 sets of doors to get through

The rewards aren't fantastic (although the pure gil that drops in to your inventory is better than a standard map) the main thing these are done for is for one of the rarest achievements in the game. (get 20 clears)

The clear rate is terrible, but the items that drop are almost completely worthless to boot now. It's mainly something for the achievement hunters