ISO 639-6 (RE: Proposed Successor to RFC 3066 (language tags))

[Harald Tveit Alvestrand]

Is there anywhere (freely accessible) a document that explains the roadmap 
and purposes of the suddenly burgeoning set of ISO 639 standards?

I understood ISO 639-1 and believe I understood the reason for 639-2.
I have a serious lack of clue about ISO 639-3, but the project to develop 
ISO 639-6 seems to also imply the existence of 639-4 and 639-5, which I 
know *nothing* about.

Can someone enlighten me/us with pointers to the description of the 
purpose, principles, timelines and responsible agencies for these projects 
- and to what degree the relevant ISO TC has committed to seeing these 
projects completed?

                 Harlad