Behavior of GetAdditionalLight

Unity Engine
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1

Hello.
I was writing a shader that only receives shadows and was having trouble with the behavior of GetAdditionalLight.
image

GetAdditionalLight has two properties, one that receives a property called ShadowMask and one that is not required.
If you pass some value of ShadowMask, GetAdditionalLight will internally set shadowAttenuation on the light, otherwise it will not set any value. Otherwise, the value is not set and the shader using shadowAttenuation receives 0 for shadowAttenuation.
I understand that the shader code is not documented and is prone to change, but is this the intended behavior?

Can get shadows from pointLight, etc.

                half shadow = mainLight.shadowAttenuation; 
Light addLight0 = GetAdditionalLight(0, i.posWS,1); // or(0,i.posWS, 0)
shadow *= addLight0.shadowAttenuation;

Cannot get shadows from pointLight, etc.

half shadow = mainLight.shadowAttenuation;
Light addLight0 = GetAdditionalLight(0, i.posWS);
shadow *= addLight0.shadowAttenuation;