I'd like to know also, because I have found it to mean two different things (or are they different?) and asked this same question before without any replies. Plasma Tungsten Arc Welding? Plasma Transfer Arc Welding?
Plasma Arc Welding (PAW) is the 'generic' term for Plasma Arc Welding. The "T" in PTAW refers to the transfer mode; i.e., the constricted arc either 'transferable' or nontransferable. So, PTAW is Plasma Arc Welding with the arc in a transferable mode. If looking in ASME IX you can see this variable defined within Table QW-257.1 (Hardfacing, Corrosion Resistant overlay, etc.) as Essntial Variable QW-410.48. I don't consider myself an 'expert' in either PAW or PTAW but have recently taken on a few jobs in our shop which necessitated learning more about the processes. Hope this helps.