They are the 'heavy-duty' modules designed to handle larger loads. These devices are superior in specification and built to the standard modules. However their prongs and socket are made for the US market, so an adapter or modification is needed to use them. To understand why, we'll need to understand a bit about powerlines and X-10 protocol. A great description of this issue is given in Phil Kingery's article on Digital X-10 at the Hometoys web library .
All utilities generate electric power in three phases.
You can see it for yourself next time you pass near a high-voltage intercity powerline. Look up and notice that the wires are grouped by triplets. Each wire in a triplet carries one 'phase'. This is repeated three times, to cover all phases as shown in the next figure. The presence of this carrier pulses indicates a logic 1, and the absence a logic 0.