The UsePass command uses named passes from another shaderA small script that contains the mathematical calculations and algorithms for calculating the Color of each pixel rendered, based on the lighting input and the Material configuration. More info
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UsePass "Shader/Name"
Inserts all passes with a given name from a given shader. Shader/Name contains the name of the shader and the name of the pass, separated by a slash character. Note that only first supported subshaderEach shader in Unity consists of a list of subshaders. When Unity has to display a mesh, it will find the shader to use, and pick the first subshader that runs on the user’s graphics card. More info
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Some of the shaders could reuse existing passes from other shaders, reducing code duplication. For example, you might have a shader pass that draws object outline, and you’d want to reuse that pass in other shaders. The UsePass command does just that - it includes a given pass from another shader. As an example the following command uses the pass with the name “SHADOWCASTER” from the built-in VertexLit shader:
UsePass "VertexLit/SHADOWCASTER"
In order for UsePass to work, a name must be given to the pass one wishes to use. The Name command inside the pass gives it a name:
Name "MyPassName"
Note that internally all pass names are uppercased, so UsePass must refer to the name in uppercase.