Simply put, fog that applies one direction, but not the other, like a one-way mirror effect, and/or fog that applies to “lower” layers but not “higher” ones.
It could be handy in examples of having creatures or players high up on cliffs, towers, etc looking down.
For example… kobolds are on a cliff ledge pelting the players with rocks or some such. They have a great view against the players, while the players wouldn’t be able to see much else except for a red head pop over before a rock falls. (Until they get to the same area, anyways)
Once the players scale to where the kobolds were; they discover theres a cave entrance on the ledge.