That’s used for Runtime Virtual Texturing specifically, there is some material setup along with asset and actor setup in the level to use it. We’ll have docs that break down both Streaming and Runtime VT at release.
For this one, you’re referring to the “From Material Expression” blend mode that can be used in a Material to have multiple shading models. When you enable this blend mode and use the Shading Model expression in the graph you can assign a shading model to parts of your graph. We’ll have a doc page for that as well at release. 