It's relatively new (last October) - stands for Avid Audio eXtension, and at least in part, is intended to streamline Avid's various formats I think - RTAS, TDM. Now there's AAX native and AAX DSP. It'll also allow Avid to transition to 64 bit processing.
I doubt there'd be any advantage for any other DAW manufacturer to adopt the standards - just a way for Avid to catch up w/ VST and AU and enter the realm of 64 bit processing.
TCB - Tea, Cats, Books...