In checking out the elevation options @ Elevation options | ArcGIS API for JavaScript 4.16 , it seems like no option would apply to elevation in MSL (mean sea level), unless it's absolute-height.
Is the absolute height baseline that of sea level based on the datum?
We do consider 0 to be sea level, but it's not based on the datum as we simplify the globe to just be a fixed radius sphere.
Not the WGS84 oblate spheroid?
No, we simulate the Earth with a uniform sphere.
