SetReplacementShader will not render objects if their original materials don't contain any properties required by the replacement shader. For example, if your replacement shader uses _MainTex, and there is an object in the scene that doesn't have _MainTex, it won't be rendered at all.

The 'tag' argument accepts the name of a tag in the 'Tags' property block of the shader. When provided, Unity will pick a sub-shader from the replacement shader that matches the value of the Tag. So if the tag argument is 'RenderType', you can provide two sub-shaders 'Opaque' and 'Transparent' in the replacement shader, and Unity will pick the sub-shader that matches the original object.

Shaders can be replaced in the SceneView using SceneView.SetSceneViewShaderReplace.