I have no idea when MP5, and MP6 rules have been made available and how much more special than the existing MP3 and MP4. It seems that when you have a gadget that supports more than MP3 and MP4, which will be crowned player MP5 and MP6 even as a player if he is able of the three, MP3 + MP4 + MP5 = MP6.
Spotted Chinavasion online shop, the gadget shown here is called an MP6 player. It is, of course, plays the usual MP3 and MP4 file formats. And it offers more for being an ISDB-T digital TV tuner and can play two additional file formats, VOB and RMVB file formats, which go with the DTV.