The classificationPrimitive affects the 3D terrain and 3D tiles as per the provided sample.
However, how to modify it to affect for custom 3D geometry ? I am adding a 3D box using viewer.entities.add() function and I require the classificationPrimitive apply on this.
It’s currently not possible to use classification primitives to classify custom 3D geometry that are not terrain or 3D Tiles. However, you can create your own primitives to classify custom 3D geometry with polygons (PolygonGeometry - Cesium Documentation).