Definition of altitude/elevation tags

We had some confusion in the German community with the ele-Tag.

Is this the “official” definition of the ele-Tag?

In Ireland it seems that the ele= tag carries the WGS84 altitude,
different from ele:local=

In Germany the ele= tags carry the local values (above sea level, N.N., NHN, DHHN92).

I think also the Wiki is not clear, because WGS84 knows two different altitude

  1. height above GRS 80-Ellipsoid
  2. height above EGM96-Geoid (corrected according to local gravity field)

So, what’s the correct definition?

As an example: NMEA defines altitude over EGM96 geoid in the
altitude field. There is a 2nd field with the geoidal separation, that
the ellipsoidal heigth can be computed easily.

We need an exact definition.