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I am trying to understand the IAP_SHADELESS node group created by the Import Images as Planes addon for shadeless materials. It looks like this

(Light Path Node docs, Ray Type docs)

This switches between an emission and diffuse shader based on the light path, similar to the popular shadeless node setup in this answer, but more complicated.

If you think for a while, you'll realize this uses emission exactly when

(RayDepth <= 1 and IsGlossyRay) or   # Camera Ray to Glossy?
IsShadowRay or                       # Shadow?
RayDepth == TransmissionDepth        # Camera or Refracted?

The third condition, RayDepth == TransmissionDepth is similar to using IsCameraRay, but works when the image is viewed through layers of glass.

The first condition, RayDepth <= 1 and IsGlossyRay, works when the image is seen in a mirror (but only one bounce?).

What is the second condition, IsShadowRay, for? Answers with a picture preferred.

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