Ah.
BMS defines certain core functionality (it can even be used for DDR songs, for example). BME adds some tweaks to basically make 7-key support possible. (I put up a summary of the basics a while ago at
http://sacredchao.net/~arashi/bme.txt a year or two ago.) There are tons of "extensions" of the BMS format that various Japanese players have made over the years, though, that I've just started learning about tonight; there are almost 60 commands that can occur in the header, and something like 80 useful play channels defined. There's even long note support for EZ2DJ-like charts, but it looks like only one player I've never heard of supports it.
BMS is pain :3 But it's cool to see people actively working on sims.