It refers to size. A2 sized bags appear to fit 
cels with oversized backgrounds and A4 are for 
regular sized cels.

That's all I can tell you. I think I've only got 
A2 myself but this info. is from Usagi's House 
which is the only place besides Noriko's that 
seems to have cel bags (I'm sure there are more 
but I'm just too lazy to look).

Wendy

Hmmm need to order a new cel book :)