For a typical climb schedule
(a) in the lower levels the aircraft maintains a constant IAS.
(b) as the altitude/flight level increases, the mach number progressively increases as TAS increases and speed of sound decreases. At some specific altitude, which is easily calculated, the desired mach number for higher altitude climb is achieved. The remainder of the climb is then done at constant mach number.
The level at which the change from constant IAS to constant mach number climb occurs usually is called the changeover altitude or level. Not specific to a particular aircraft - only the selected IAS and mach number.