A 'CoShader' provides arbitrary input values to another shader but also a full fragment Pixel output which allows easy chaining of multi-channel shaders together which is common for Nuke.

Normally there is only one surface shader per shading network which may an arbitrarily-large coshader input network feeding it. Surface shaders apply lighting calculations and are the final shading point before fragment integration.

See SlrSurfaceShader and SlrLightShader for surface & light shading specifics.