What is the difference between ellipsoidal height and elevation?

Elevation (using EGM96 model):

Elevation (positive height):
The positive height of a point on the Earth's surface is the distance from that point along the plumb line to the geoid; in fields other than geodesy, the positive height is also referred to as the elevation. The positive elevation of a point is related to the difference in potential energy between the level over the point and the starting geoid, does not vary with the leveling route, and can be uniquely determined so that it can represent the elevation of a point. A positive elevation system must be associated with a geoid whose gravity level is known, and this gravity level is the geoid gravity level. The geoid is the reference surface of a positive elevation system.
The EGM96 model is a high-precision geoid model introduced by the United States, which is applicable to the global scale and is calculated by comprehensively utilizing a large amount of existing global gravity data. The model can be used to solve the geoid gap at any point in the world, and its accuracy reaches the centimeter level within a range of 50 km in the United States mainland.

Geodetic height (WGS84 ellipsoid height):

When performing calculations of spatial position (e.g., calculating the position of satellites in global satellite navigation systems such as GPS), the height used is then the distance relative to the reference ellipsoid surface, called the geodetic height or ellipsoid height. The conversion between the two needs to be done via the geoid height.
The reference ellipsoid is a mathematically defined surface of the Earth that approximates the geoid. Because of its relative simplicity, the reference ellipsoid is the geometric model of the Earth's surface preferred for calculating and displaying point coordinates for geodetic control networks. What is commonly referred to as the shape and size of the Earth is actually expressed in terms of the long half-axis, short half-axis and oblateness of the reference ellipsoid.
Geoid height, also known as geoid rise or geoid gap, refers to the distance from a point on the geoid projected along the normal to the surface of the reference ellipsoid.
When the geoid height is positive, it means that the geoid is above the reference ellipsoid, and vice versa, it means that it is below the reference ellipsoid. In the orthometric system, the geoid height is also described as the difference between the geoid height and the orthometric height.

Conversion calculation relationship between ellipsoidal height and altitude:

• Due to the different terrain undulations, the difference between elevation and ellipsoidal height varies in different regions. This difference is called the altitude anomaly. The conversion formula is very simple. Users can simply add or subtract if they know the altitude anomaly value. Altitude anomaly value: the height from the geoid to the Earth ellipsoid. Altitude anomaly values can be obtained from the national mapping department. Formula: ξ=H-h, where H is geodetic height, and h is normal height/altitude. In MSDK v5, we provide a tool class interface GpsUtils package.

//To initialize, you need to use GeoidManager.getInstance().init() first, and then use the interface of GpsUtils.
//Elevation TO Geodetic height
public static double egm96Altitude(double wgs84altitude, double latitude, double longitude)
//Geodetic height To Elevation
public static double wgs84Altitude(double egm96Altitude, double latitude, double longitude)