There is a desire to register language features, like go to definition, for a specific language but with low confidence score (because the provider isn't 100% correct in how it computes its results). Today this cannot be expressed - the * document selector is too specific because it applies to all documents and a selector like { language: 'typescript' } cannot say anything about its confidence. Today, the * selector often has a implicit/assumed lower confidence score and is used like that. We can now extend the selector with a notion of "specific language, low confidence" or allow to express result confidence in the returned data.