What is the default vertical datum of CesiumJS?

I am wondering, if there is a default vertical datum in CesiumJS and which type is it?

I think this thread answers your question and I found it a useful discussion, particularly Kevin’s response about heights in Cesium being relative to the WGS84 ellipsoid:

https://groups.google.com/d/msg/cesium-dev/Xz6_FnCuGNA/1bhjDugxQw0J

Thank's a lot!