Orthometric vs ellipsoidal height

The orthometric (geoid) height of a point of the Earth Surface is the distance H_o from the point to the geoid.

The ellipsoidal height of a point of the Earth Surface is the distance H_e from the point to the ellipsoid.

Earth surface, ellipsoid and geoid	Orthometric height Ho and ellipsoidal height He

The geoid height above the ellipsoid (N) is the difference between the ellipsoidal height and orthometric (geoid) height.

N = H_e − H_o

Where:

N = geoid height above the ellipsoid
H_e = ellipsoidal height
H_o = orthometric (geoid) height 

More information on this topic can be found in the following articles:

When to use the Geoid Height Above the Ellipsoid function

How to define Pix4D outputs with respect to a geoid model