Since the Tweak Workspace is only responsible for containing and manipulating polygonal mesh data, in that context it is only appropriate to export that data as mesh data in one of the following formats - .obj, .lwo, .fbx, .stl, and .ply
When any Retopo Group is “Merged” into the Paint Workspace, it also makes its appearance into the “Objects” Panel - which is the main Panel associated with the Tweak Workspace.
When you use the “File/Export model” option, all of the Objects which are present in the “Objects” Panel are exported, whether they are currently visible, or not.