Wyvern

its your true speed
Airspeed usally gets displayed in either indicated or true 
Indicated is how fast the air is flowing past your plane and that can change depending on your altitude. Its useful for maneuvering and such.
True is how fast you are actually moving.
so at around 30000ft your Indicated vs True speed is something like 500I vs 800T, 

The reason for that is because the air at higher altitude is generally thinner, meaning when your plane is measuring how much air is flowing past your plane, it will pick up less air and think youre going slower.
(its a bit more complicated, but that is enough to understand the concept)
Most planes do have the ability to calculate the actual speed (which also is why mach is not bound to your altitude)

NOW, the reason why the Su-27 is showing your airspeed in True is because youre in BVR mode.
You dont need to know how fast the air is moving around you, since youre likely moving rather fast already, and you need to know what your actual speed is, as that is also important for a missile launch (it also plays into closure rate to your enemy and and and)
When you go into Nav, Vertical Scan, HMD, or HUD BFM, or ground mode, it will change to Indicated speed (123I), as you need to know how well you can maneuver.
 

Its due to him not having the mod. As his game isn't property showing the missile.

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