[tei-council] Extended language subtags must begin with the letter "s".???

[Martin Holmes]

CH says:

"Extended language subtags must begin with the letter "s".
  They must follow the primary subtag and precede subtags that do
  define other properties of the language.  The order is significant."

I can't find any justification for this in BCP 47 or anywhere else, and 
it's surely untrue -- . Does anyone know where this came from? Basic 
examples from BCP 47 surely counter it, too:

"zh-gan", "zh-yue", "zh-cmn"

I'm copying Syd on this because he knows a lot about this stuff.

Cheers,
Martin
-- 
Martin Holmes
University of Victoria Humanities Computing and Media Centre
(mholmes at uvic.ca)