Voluntary Application Server Identification for Web Push
An application server can voluntarily identify itself to a push service using the described technique. This identification information can be used by the push service to attribute requests that are made by the same application server to a single entity. This can used to reduce the secrecy for push subscription URLs by being able to restrict subscriptions to a specific application server. An application server is further able include additional information the operator of a push service can use to contact the operator of the application server.
https://datatracker.ietf.org/doc/html/draft-thomson-webpush-vapid