FGeoSelectionID provides a pair of 32-bit unsigned integers that can be packed into a 64-bit unsigned integer for use with FGeometrySelection. This is generally intended to be used to encode a mesh geometry ID (eg TriangleID, EdgeID, VertexID) combined with a "Topology ID", eg something like a Face Group ID. However none of this is enforced and so a caller can use these two integers for any purpose.