[Dovecot] What does "a" flag in Maildir format mean?

Here is sample filename from Maildir:

1243423383.M745917P32169.termserv1,W=1295:2,Sa

1. why there is no S=<size> field? It definetely differs from 1295.I've not quota plugin, could it be the reason of this?
2. what does "a" flag mean? I not found any description of this neither in Maildir spec nor on the Dovecot wiki.