The RenderPipeline tag tells Unity whether a SubShader is compatible with the Universal 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
See in Glossary (URP) or the High Definition Render Pipeline (HDRP).
This example code declares that a SubShader is compatible with URP:
Shader "ExampleShader" {
    SubShader {
        Tags { "RenderPipeline" = "UniversalPipeline" }
        Pass {
            …
        }
    }
}