I'll have a think about writing a short knowledge base article about formats such as m4p. A lot of people are unaware about the concept of "DRM" (Digital Rights Management) and how it affects what, where and how they can listen to music they buy online. It's a massive problem, but it basically comes down to the online stores not wanting to get massively sued by the RIAA. The end result is the user gets shafted when they find out their purchased music can't play on their favourite mp3 player, nor can they convert it to their favourite format either.