Inputs
Here you can set a link to the Node material you want to edit. To do so, simply drag the material from the Material Manager into the Node's Material field.
Here you will find all inputs that the linked material has. In order for this list to appear, you have to click on the Update Material Parameter Ports button after the material has been linked. The material parameters must, however, be set up as inputs on a material group and this group must be defined as a Start Node. The Uber material will meet this requirement by default.