It's for NOAA's GNOME oil modeling tools, specifically the web frontend.
https://gnome.orr.noaa.gov/
Here's an example of the map view so you don't have to figure out how to get the data and set up a simulation. The yellow polygons are what the simulation sees as 'land'. However, these land polygons do not have the lakes integrated, those are separate polygons with a different classification.
https://photos.google.com/share/AF1QipOMbvwbkbFTFIyNPZHjgZLYFqIczAol8uv9S5rvheDFPVSPT6z5ZEwr-bkdMzzMuQ/photo/AF1QipPdrZd00GhOBFOC5QDObNyUbr7c1YajSnNfbVbq?key=Q2dqZjF0WW9SNXFPeEdhQkpLMG1IMlVIWDhQM0dR
Basically, we wanted the lakes to 'look like' the sea, where they would be transparent to whatever base layer put below. In the past, the best we could do was color them differently or not draw them at all.
There's actually quite a bit of low level Cesium work in the various bits of that frontend, mainly in the pursuit of speed with huge datasets and rendering the simulation output close to real time. Now that the Map is converted to use Primitives I think we aren't using Entity level abstraction anywhere.
By the way, I made a further modification to the Sandcastle, where I added another separate blending object to the circle's RenderState so I could experiment a little. It appears that if the'translucent' appearance option is false, it does not get a blending option and becomes unaffected by the mask. However, I found you can still add a custom blending object and leave it disabled, which makes sense. The question now is whether I can enable the blending object and set some combination of settings to evade the mask. Again, I think I need to do some learning and truly understand what's going on in the graphics stack before I spend a lot of time guessing.
https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=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