Can some one in the “know” put some numbers on these terms. At how many volumes does petillant become sparkling?
Well, I may not be in the know and I do not have any specific numbers but here is my take on it ( that will likely spur someone in the know to reply ;))
Still is still. Dead flat, no bubbles.
Petillant just slightly bubbly. Generally I will call a cider petillant when I intended it to be still but, despite my best efforts to wait MANY months for fermentation to be completely done some slight fermentation occurs in the bottle anyway. 
It will fizz very slightly as it is poured and there will be very gently carbonation. Just a slight mouthfeel but not too prickly of a sensation. If I had to guess numbers I would say a quarter volume up to just under one volume?
Sparkling. Well generally I would say champagne levels. Highly carbonated, very prickly bubbles. 3-4 volumes? I guess it depends on what we are talking about though. A sparkling wine may be more carbonated than a sparkling cider. The liquid itself may give a different perception depending on the body and residual sugars (or dextrins where appropriate) that are there.