[asterisk-users] weird undocumented extensions such as s-BUSY

[Barzilai Spinak]

I've seen several examples that use extensions such as;
s-BUSY
s-NOANSWER

etc...

It's more or less evident what they do, but I've searched for some 
FORMAL documentation everywhere and have found nothing.
Do they work for anything else than "s-"? (I think I've seen other 
examples, but can't find them now)
Are they standard in any way?
What are the allowed values after the dash?
In which version were they introduced?
etc...

(please no replies explaining me how "s-BUSY" matches when the start 
extension is set busy or trivial explanations like that)

BarZ