There are 2 fogs you can use, atmosphere fog and a fog plane. The fog plane has to go in a strait line and can not wrap around. It's most commonly used for water. When your in a cluster with a fog plane assigned to it, you will see fog everywhere at the level with the fog plane. However if you had a cluster right next to it without fog assigned to it, you won't see any fog even where it is supposed to be in the other cluster.