TMS transparent backgrounds

Why does the CesiumTileMapServiceRasterOverlay added by Cesium World Terrain not support transparent backgrounds,
but added by Cesium3DTileset supports,
the same resource.
I want to add a TMS with transparent background in Cesium World Terrain.

The alpha channel of a TMS layer is blended with the layer below it. For the bottom-most layer, that means the color of the model itself, which for Cesium World Terrain is plain white. If you want different behavior, you can achieve it by creating a custom material.

Thank you for your reply,
Is there any “custom material” here that I can refer to?

I don’t know of any already-made material that does what you want out of the box. But this tutorial walks you through the process of creating a custom material: