I am currently working on a project with clipping planes and point clouds, but I think this question applies to planes in general:
If I understood planes (clipping planes) correctly, they consist of a normal and a distance to the origin. Also they are part of a collection, where the logic of all planes is kept and they can be added to different viewer entities. Internally, the planes are infinite, while the viewer entities have finite dimensions.
The problem I have is now: I want to rotate the plane by using HeadingPitchRoll. For Operations, where the plane's normal is changed, this works alright, but if I try to rotate it around the normal as axis, nothing changes in the appearance. I solved this problem by using a polygon instead of a plane entity in the viewer. But I'm still wondering, if there might be a better way with the use of plane entities.
It seems as if the plane-entities always orient themselves along their original axis inside the plane they describe and not in relation to the transformation their modelMatrix describes.
I tried to visualize this in a modified version of the "planes" sandcastle:
As you can see the two planes are oriented the same way, even though the second one should be tilted around the y-axis.
I am currently using Cesium 1.45.