It could be a lot of things, basically anything where part of the computation is offline or cached.
The baked direct shadows do make sense. Basically the current stationary lights except only the shadowmaps, no lightmaps (the GI). I even think it kind of already works, probably accidentally.
They would be very useful for all artificial lights (so anything but the sun), because they don’t change position (so shadowmaps would stay the same), but it may turn off/on/change intensity (which you can currently do on stationary, but their GI will stay the same).
The benefits (compared to fully dynamic) would be improved performance. Currently on a fully dynamic lighting scenario, shadowing will be a good chunk of total frame time, plus require a lot of draw calls. That’s regardless of using lumen or not. Plus stationary lights can have better shadow quality than movable if it’s a soft light (although not as good as RT shadows)
And I think you could have almost real time iteration time. Because you don’t need to calculate the GI for lightmass (the expensive part), and the direct shadowing is very cheap to calculate I believe.
To be fair this has some overlap with shadowmap caching for movable lights, but I found these to be not that impactful because they would get constantly invalidated I guess.
The lumen reflections for lightmass would be cool, because, it would be (I guess) more performant than RT, not require RT, and less noisy on rough materials, which RT struggles on unless you crank the settings and kill performance. Basicallly improved cubemaps if you will (because instead of SSR falling back to cubemaps due to roughness or off screen, it would fall back to lumen (techically lumen itself handles the SSR)).