6th Face refers to the face opposite the main machined face of a part.

For example, in a Door the main face (1st face) may be the front of the door with  a routed design. The 6th face being the back of the door, where the hinges are mounted. The reason it is the 6th face is that the four edges of the door are faces 2 to 5. So when processed on a flatbed, the part has to be turned over to machine both its 1st and 6th faces.

6th Face Machining

The My Setup > 6th Face Machining page in the Catalog/Drawing Properties allows you to enable 6th face machining.