As a (very rough) rule of thumb we fly relative to indicated airspeed in knots up to approx FL280 to FL300 and then Mach number above that. Consequently each jet airliner has 2 maximum speeds for their normal operation – Vmo (mx operating speed) and Mmo (max operating mach number). For the A340 Vmo is 330 kts and Mmo is M.86. Therefore up to FL 290’ish we would be limited to 330 kts but above that the controlling speed limit is M.86. The actual altitude that the changeover occurs at is the altitude where the temperature is such that M.86 equates to 330 kts.

One thing that needs to be borne in mind is that when the aircraft reaches high altitude the air molecules (which is what the instruments sense) become much more spread out (lower pressure) so instruments tend to massively underread. This is the difference between Indicated airspeed and True airspeed (any professionals please give me a little latitude in simplifying this explanation!). Consequently an indicated 330 kts at altitude might actually be a true airspeed of over 500 kts. This is how an aircraft can actually fly (‘true’ speed) much faster than Vmo (which is an ‘indicated’ speed).

As you can probably now see, it’s much easier to use Mach numer (easily simplified by pretending that the numbers are a percentage of the speed of sound) as a speed reference when in the higher parts of our atmosphere.