Recordare has released a new Beta 5 version of the MusicXML 3.0 format. You can download everything from the zip file at:
http://www.musicxml.org/dtds/3.0/musicxml-30-beta-5.zip
Beta 5 has many documentation improvements. We have specified and clarified several parts of the format in response to developer requests collected over the past few years.
This may be the last beta version before MusicXML 3.0 is released. Please let us know if there are things we should be adding or changing to better meet your needs. The MusicXML mailing list (www.recordare.com/musicxml/mailing-list) is the main place for for these discussions, but please comment here or email feedback privately if you prefer.