By "registration point" I'm guessing you mean 'origin'. Yes, 3D meshes do have origins. In fact, all point(vertex) coordinates in the mesh are relative to this origin. For most meshes the origin is usually smack-bang in the centre but it can actually be set to *anywhere* in 3D space.
If you position a mesh with PositionEntity, the point in 3D space you specify is where the mesh is placed, with it's origin at that point. And, yes, EntityRadius etc. are also set at the origin.
If you position a mesh with PositionEntity, the point in 3D space you specify is where the mesh is placed, with it's origin at that point. And, yes, EntityRadius etc. are also set at the origin.