The real VNE is a TAS, however the marking on the ASI is an IAS. In small piston engine airplanes it is apparently normal to take the lowest IAS corresponding to the real VNE and mark that on the ASI which should usually keep one save, however if you climb higher than the normal service ceiling (using updrafts, mountain waves for example) you might be in for a rude surprise if you try the marked VNE.
Since gliders do not have a service ceiling they simply mark the VNE for sea level and give the pilot information how it changes with altitude in the POH and/or a placard on the panel.