Where is the source for Transparent/Diffuse/FORWARD shader pass?

Transparent/Diffuse/FORWARD shader pass is referenced in the docs, in the "Transparent/Diffuse ZWrite" example for Transparent shader with depth writes:

I cannot find that shader pass anywhere in the shader source code. I downloaded the latest shader source version "builtin_shaders-5.0.0f1" from the Download Archive here, but cannot find it. I tried several older versions as well.

I did find the "Legacy Shaders/Transparent/Diffuse" shader, but there is no pass named FORWARD. Also, if I replace the UsePass line in the "Transparent/Diffuse ZWrite" code with the actual code wrapped in CGPROGRAM/ENDCG from the Legacy shader, I get a different result, which indicates to me that it is running different code.

Any idea where I can get the source code for whatever Transparent/Diffuse/FORWARD is referring to?