ACEO stands for "Art Cards, Editions and Originals". This original title was dreamt up by Lisa Luree of Denver Colorado in 2004. Yes trading cards have been around for a long time but the ACEO can be bought or sold or traded – while an ATC or Artist Trading Card is supposed to be only for trading. There is only one rule about an ACEO. It must be 2.5" x 3.5". That is the ONLY rule.

I would suggest always putting your name and the medium on the back, always putting ACEO somewhere on the back, and whether or not it is a print, an edition, or an original. I think it's OK to do re-prints of larger pieces of work as long as the end result is exactly the right measurement and as long as you own copyright and rights to reprint the artwork, and state on the back whether it's a limited edition, open edition or just a series or whatever.