When writing Surface Shaders, you’re describing properties of a surface (albedo color, normal, …) and the lighting interaction is computed by a Lighting Model. Built-in lighting models are Lambert (diffuse lighting) and BlinnPhong (specular lighting).
Sometimes you might want to use a custom lighting model, and it is possible to do that in Surface Shaders. Lighting model is nothing more than a couple of Cg/HLSL functions that match some conventions. The built-in Lambert and BlinnPhong models are defined in Lighting.cginc file inside Unity ({unity install path}/Data/CGIncludes/Lighting.cginc on Windows, /Applications/Unity/Unity.app/Contents/CGIncludes/Lighting.cginc on Mac).
Lighting model is a couple of regular functions with names starting with Lighting. They can be declared anywhere in your shader file or one of included files. The functions are:
half4 Lighting<Name> (SurfaceOutput s, UnityGI gi);
This is used in forward rendering path for light models that are not view direction dependent (e.g. diffuse).
half4 Lighting<Name> (SurfaceOutput s, half3 viewDir, UnityGI gi);
This is used in forward rendering path for light models that are view direction dependent.
half4 Lighting<Name>_Deferred (SurfaceOutput s, UnityGI gi, out half4 outDiffuseOcclusion, out half4 outSpecSmoothness, out half4 outNormal);
This is used in deferred lighting path.
half4 Lighting<Name>_PrePass (SurfaceOutput s, half4 light);
This is used in light prepass (legacy deferred) lighting path.
Note that you don’t need to declare all functions. A lighting model either uses view direction or it does not. Similarly, if the lighting model only works in forward, do not declare the _Deferred or _Prepass function. This way, all Shaders that use it compile to forward rendering only.
Similarly, customize the decoding lightmap data and probes by declaring the function below.
1.half4 Lighting<Name>_GI (SurfaceOutput s, UnityGIInput data, inout UnityGI gi);
Note that to decode standard Unity lightmaps and SH probes, you can use the built-in DecodeLightmap and ShadeSHPerPixel functions, as seen in UnityGI_Base in the UnityGlobalIllumination.cginc file inside Unity ({unity install path}/Data/CGIncludes/UnityGlobalIllumination.cginc on Windows, /Applications/Unity/Unity.app/Contents/CGIncludes/UnityGlobalIllumination.cginc on Mac).