Hey, Airfoilmod--not exactly--I'm saying different part become sonic at different speeds-high perf.aircraft..DC-9s, P51 Mustangs, F18's will achieve upon parts of its airframe both regions of sonic and supersonic flow...as long as they fly near sonic velocities...-- --the main reason [regarding the wing] is sweep which decreases the length of the velocity vector along the wing-----an important concept is the M-crit which is when the very first supersonic flow is encountered on the airframe--[they learn this i n the wind tunnel]---it is similar, in concept, to a propeller tip or chopper blades where [in this case ] the angular velocity is higher at the tip than the roots--as a result---speed is limited because the supersonic flow creates a good deal of drag--
Well because of this all of the defined regions have a value higher than the minimum--to ensure that the whole airframe is 'immersed' in a sonic/supersonic or hypersonic flow---
PA