As Pilot-DAR says, Vne is marked as
IAS - just like everything else on the airspeed indicator. All the speeds on the ASI are
indicated airspeeds - obviously
It is some of the factors which give rise to Vne that are related to TAS and not IAS - control surface flutter I think being the main one.
The amount by which TAS exceeds IAS is altitude dependent, so the ASI marking for Vne takes into account the aircraft operating ceiling - because you could legitimately climb to that altitude and then immediately commence a dive at Vne.
This causes problems if you fit a more powerful engine (e.g. as in turboprop conversions) because you can now fly higher, where the TAS gain is greater that the figure used for the original ASI marking for Vne.
So, on straight TP conversions like e.g. the Jetprop, the certification forces Vne to be marked at a lower IAS figure than on the original aircraft (piston Malibu).
At least that is my understanding