Left Toolbar > Project Nodes > Material IO
The Material IO node defines an input or output of MATERIAL node value type in Custom nodes.
On the left toolbar choose Project Nodes > Material IO
Drag the Material IO icon into the Node Graph.
Connect the input: material.
Connect the Output: material.
material
The material value that will be received by the custom node's node graph as input. Material IO nodes that have connected outputs are used by the Node Graph node as inputs in the custom node.
material
The material value that will be returned by the custom node's node graph as output Material IO nodes that have no connected outputs are used by the Node Graph node as outputs in the custom node.
Loop Action The type of action applied on this image node value after each loop:
replace: Replaces the previous value with the new value.