This is a dimensionless factor, the plane speed divided by the sound speed, it's the norm of the speed from 3.0 fg version, was unclear before.
- The pitot tube use this to give dynamic pressure.
- It's used in some planes to display the Mach speed, i recommend to make them use:
Conflict - solution
Solved YASim was giving the longitudinally projected Mach velocity, and JSBSim the norm of the Mach vector. this give false airspeed when alpha or beta are not null. the retained solution was to use the Mach norm for both, with some change in the pitot code.