Custom Fabric Shader

Hello Everyone,
I am fairly new to Unreal and have been following a document on how to build Custom Fabric Shader. However since this is my first time in the Material Function and am not familiar with all the nodes, I am unsure what some of the nodes are in the diagrams.
I was wondering if someone could help me figure some of this out. I am currently stuck on page 17 and have every node in place except for the node marked “LightVector” can someone tell me what node that is?