Yes, it has a lot to do with Vne. It is the place where the stall speed
runs into *something* limiting the high speed. The upper speed limit is
Vne by definition, whatever the cause of this limit may be. Note that
Vne may vary with altitude, temperature, whatever. This red line is just
a simplification.

The mach number matters because in real life, the coffin corner was
relevant to military aircraft only. Their Vne was typically set by mach
speed.

Stefan


I think now you splitting hairs. But if you really want to, Vne does not
even apply to most aircraft for which the "coffin corner" is an issue. The
"red line" on the airspeed indicator is not fixed and moves, it is the
lesser of Vmo or Mmo, which is the maximum operating speed (akin to but not
technically the same as Vne) or mach number.

Ivan