How do I get fog in just one spot: like *fog*>*no fog*<*fog* ?

In your map, you must create plane objects where you want the 'boundary' between your fog zone and fog-free zone to be. Then, in your material, create a new sub-material. Don't assign a bitmap to it; instead, call it '+unused$'. The +unused allows the material to parse through tool without the need for it to have an assigned bitmap (I think), while the $ sets up the plane as the marker for a fog plane volume.

Set up as many of these planes as you want, but make sure they completely 'intersect' your map, meaning there are no gaps that the player can access/go through, i.e. see a sudden cut in the fog, for aesthetic reasons. It makes no sense to have the plane set up to cover part of the map, with some part un-fogged in the same plane.

From within Sapien, you have to instantiate the fog planes (or something. I myself have never performed this before, so I am not entirely sure how to accomplish this).