"a" is for "alpha", "b" is for "beta". Wikipedia probably defines those terms nicely but, in brief, "beta" means "almost release quality", and "alpha" means "experimental quality (ie. buggy)".

To answer the question more fairly, v3a11 is a regression (w.r.t. sigkills et al.) because it involved a major shift in the underlying code base -- massive changes.

Cheers