Senior flex nonsense

It seems that the designation for senior flex shafts has gone from S to A. I believe this is so the flex is not confused with S as in STIFF flex. But what does the A stand for?

Ancient?
Antique?
Arthritic?
Alzheimers?
Aged?
Abject?
Abominable?
Ailing?
Asinine?
Awkward?

It must be one of these or maybe ALL of them/