Oh its just what the struct containing the lighting data is called. It's a custom struct declared in the UE base pass where lighting gets collected before running the material graph. I've got a write-up explaining how it works

dev.epicgames.com/community/le...

That HLSL node I thinks always been there, but it got an update in 5.4 with just nicer UI and syntax highlighting.

Though there's a cool plugin made by Phyronnaz that generates material functions from HLSL files, we have a fork of it that extends it to full materials

github.com/Abdul-Nosirr...