At lower levels, the limiting consideration is airloads on the aircraft ... EAS things, so the pilot has an IAS limit.
As the altitude increases, for a given climbing IAS, TAS increases, OAT and speed of sound decrease .. so the Mach Number increases. At some particular height/IAS combination the Mach Number is high enough so that, for the particular aircraft, compressibility considerations become more limiting than airloads.
Below this height, IAS reigns, above Mach Number.