ATC is a fairly common and widespread one (ie it's used across the art community) to mean Artist Trading Card.
TAST is a 'Sharon-isn' (!!!) It's short for the Take a Stitch on Tuesday challenge that she ran through her blog etc a couple of years ago and that is being re-done (I think) by the group of the same name on here. I doubt you'd find that on one of the stitchers' acronym lists that exist here and there.
Hope that helps. Any more you want to know about?=)
ATC certainly means Artist Trading Card, which are made to be traded with other artists, not sold, but I have just seen some advertised for sale on Etsy!
ATC is the abreviation for Artist Trading Card, which should measure two and a half, by three and a half inches.
They can be composed of paper, textiles or any mixed media you wish to use(there may be more rules, but you would have to check with individual groups to clarify this).
Hope this helps a little, I'm sure there are more knowledgeable people in the group who could help with more information!