Actor Mobility is unclear

I’m confused about the mobilities I can set for a StaticMeshActor.
Here it says:

Static Mesh Actors cannot have a Mobility of Stationary.

So I realized a StaticMeshActor can only be static or movable…
But why is it then possible to set it stationary? Hovering over the stationary button says, that the object can be changed but not moved and enables cached lighting methods “Cached Dynamic Shadows”.
But I cant find anything about “Cached Dynamic Shadows”…
Until now i couldn"t find any difference between setting a StaticMeshActor stationary or movable…