Tags are key-value pairs of data. Unity uses predefined keys and values to determine how and when to use a given SubShader or Pass, or you can also create your own custom SubShader and Pass tags with custom values. You can access SubShader and Pass tags from C# code.
The most commonly used predefined Pass tag is the LightMode tag; this is used in all render pipelines. Other Pass tags vary by render pipelineA series of operations that take the contents of a Scene, and displays them on a screen. Unity lets you choose from pre-built render pipelines, or write your own. More info
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