The nature of air flow changes at the speed of sound, hence the sonic boom. For subsonic aircraft you don't want any sonic boom anywhere so it is important to know how close to the speed of sound - Mach 1.0 - you are, and typically less than 0.9 to be sure no surfaces are in sonic boom territory.
The characteristics for supersonic flight change at faster than the speed of sound.
The speed of sound changes with air density and pressure so using air speed alone is not a good indicator.
Note 1: All this is as simple as I can make it
Note 2: Sorry to the OP if I've missed the point.
Note 3: Web search is your friend.