This node is a way to add Nested Dielectrics support to a MaterialX surface shader. The MaterialX shaders don’t have this ability built-in like the Principled Shader does.
Adding nested dielectrics is done by inserting this node between a surface shader and the material output.
You will likely want the IOR on your Surface Shader and this node to match. One way to do this is using a Parameter VOP to drive both inputs.
See Nested Dielectrics for more info.