IAS starts at about zero on reentry, and gradually goes up from there

To add on what this person said you'll hear them say when the vehicle is supersonic and when they reach max q (max dynamic pressure). Mach number is important until you hear them say "Vehicle is supersonic." and sort of important until you hear "Max Q". The mach effects vary gradually and abruptly. Abruptly because a large drag rise and stability change happens as you cross the speed of sound. There's more about shockwave propagation and reflection which I typed about below, but that's me self-satisfying my inner nerd. So I deleted it. The gradual variation from there has to do with the gradual way the atmosphere thins out with altitude.

It never becomes "meaningless" per se at a certain *altitude*. Once you enter LEO, you need to be at a certain speed to maintain orbit. Once you accelerate to this speed, you no longer need to add thrust - around Mach 25, or 17500 mph, and the astronauts earn their "Mach 25" patch. At this point, it doesn't really have any meaning anymore. For the astronauts who serviced Hubble, which is further from the Earth than normal LEO, they actually reach Mach 26 on their return to Earth, so they get "Mach 26" patches. To re-enter, they only need to reduce their orbital velocity by about 200mph, so I guess their exact speed technically matters at this point as well.

Speed limits are derived from q-bar limits. In the case of the Space Shuttle, it had a max limit of 819 psf, which equated to a maximum structural speed of 492 knots equivalent airspeed (KEAS) the mach transition equates to around Mach 1.5, at this point KEAS decreases as Mach increases.

The flight planning and thrust reductions kept the maximum speed on ascent below 475 KEAS again to minimize the maximum structural pressure to remain below 819 psf.

On descent, the maximum airspeed (KEAS) was set at 321 knots or a maximum pressure of 350 psf. "Redline" in that context was established at 333 KEAS for the Space Shuttle.

Minimum controllable airspeed (Speed that flight controls remain effective) is 163 KEAS and less than MACH 5.0.

L/D for the Shuttle occurred around 190-220 depending on MACH at the time. They always flew faster to retain energy reserves to extend the glide if needed.

Yeah, well, it's all fun and games until someone applies the brake.