How to interpret elevation data?

It is important to know that elevation data provided states the difference between the WGS84 ellipsoid and the point on earth surface, as you can see in image below from https://support.virtual-surveyor.com/support/solutions/articles/1000261349-the-difference-between-ellipsoidal-geoid-and-orthometric-elevations- where it is possible to find more information on this topic.
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Image source: https://support.virtual-surveyor.com/en/support/solutions/articles/1000261349-the-difference-between-ellipsoidal-geoid-and-orthometric-elevations-

So, if you wish present elevation with relation to mean sea level you should do a egm96-relative to WGS84 ellipsoid-relative conversion, you can use for example NPM package: https://www.npmjs.com/package/egm96-universal