"no" vs. "nb" and "nn"

[Deborah Goldsmith]

I can't find a definitive statement anywhere on the relationship  
between "no" on the one hand, and "nb" and "nn" on the other. Is "no"  
a fallback for "nb" and "nn", the way "en" is a fallback for "en-US"  
and "en-GB"? Is "no" simply obsolete? Is there some other  
relationship? What is considered the best common practice today?

Thanks,

Deborah Goldsmith
Apple Inc.
goldsmit at apple.com