How to fit Unity’s plane with Cesium World Terrain
It’s hard to understand what are you trying to do – can you elaborate on your use case? Cesium World Terrain isn’t a flat surface, so it won’t perfectly align with a flat Unity plane.
Unfortunately, Cesium for Unity does not have the exact same features as CesiumJS, and it doesn’t currently support shapes that can be draped over terrain. But it may be possible to achieve something similar using a material. You can make it recolor fragments that are inside some rectangular bounds.