At a particular Mach number, a fluid behaves similarly, but many other conditions could be different.
For example, an aircraft flying at Mach .85 at a low altitude will be in denser, warmer air, and actually flying faster, than the same aircraft flying at Mach .85 at a high altitude.
What aerodynamic differences (if any) will the aircraft experience between the two conditions?
As far as flight aerodynamics is concerned, which of Mach number, true airspeed and indicated airspeed are most significant, and when?