Wall socket, optional receiver and module are all X-10 receiver. The differences are: wall socket are mainly installed on the wall, and it appears like the traditional socket; optional receiver is the one which can be used once be plugged; If you plug it optional on the socket and then connect the lightings or electric devices, it can control the lightings or electric devices; while the module, which is concealed receiver, has small bulk, so it is usually connected directly to the lightings or electric devices.