Dynamic height is constant if one follows the same gravity potential as they move from place to place. Because of variations in gravity, surfaces having a constant difference in dynamic height may be closer or further apart in various places. Dynamic heights are usually chosen so that zero corresponds to the geoid.
When optical leveling is done, the path corresponds closely to following a value of dynamic height horizontally, but to orthometric height for vertical changes measured on the leveling rod. Thus small corrections must be applied to field measurements to obtain the orthometric height usually used in engineering. US National Geodetic Survey data sheets  give both dynamic and orthometric values.