It was added to the F1 regulations in 2011 so it's not too new of a concept. I've only ever heard the '2 wheels' rule in sim racing but then again I haven't been following racing for very long.This is substantially changing the definition since the concept of two tires remaining on track does not even appear in it. However, , it does clarify that slightly stepping on the white line is considered that the tire is in the track.
Ultimately the officials of the event can redefine the track limits, some more strict and some less strict as Whiting does for the F1 and S397 have here.