What does 'SetTexture[_]' mean in a shader?

I’m looking at a shader and trying to figure out what it’s doing, and I see the following line:

SetTexture[_] { ..... Some combiner }

What does the ‘_’ refer to? I can’t find any reference to it in the shader docs.

Found an answer somewhere else. Apparently “SetTexture [_]” just means you don’t care about binding to an actual texture, you’re just going to do another combiner stage using the previous result.