After all Subshaders a Fallback can be defined. It basically says “if none of subshaders can run on this hardware, try using the ones from another shader”.
Fallback "name"
Fallback to shader with a given name or…
Fallback Off
Explicitly state that there is no fallback and no warning should be printed, even if no subshaders can run on this hardware.
A fallback statement has the same effect as if all subshaders from the other shader would be inserted into its place.
Shader "example" {
// properties and subshaders here...
Fallback "otherexample"
}