Most higher performance aeroplanes have both an IAS and an IMN limit.
At low altitudes, invariably Vne > Mne.
If you climb at a fixed airspeed, the Mach number rises as density and temperature drop. Eventually the Mach number will hit Mne without the IAS ever having increased.
Above that speed, you have to start staying below a limiting Mach Number, which will be below Vne.
The altitude where you switch from worrying most about airspeed to worrying most about Mach number is that where Mne = Vne.
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