Creating Material Definition Asset
Click ADD NEW button.
Hover mouse cursor on Reality then select "MaterialDefinition".
Select MaterialDefiniton class.
Node and Category Name
Open your MaterialDefinition asset.
Node name is defining your nodes name and the Category name is defining your nodes location in the Reality Setup.
Click add button and then choose the variables type that you have declared in the material editor.
The PARAMETER NAME has to same with parameter name of material nodes.