Here’s a cheat sheet:
StaticMeshActor = Unbreakable, Unbuildable collision
FortStaticMeshActor = Unbreakable, buildable collision
Build-able collision means you are able to build within the FortStaticMeshActor, where as if you had a StaticMeshActor it will block you from building.
They are many other differences as they have different purposes/functionalities