IETF RFC 8292 PDF

An application server can use the Voluntary Application Server Identification (VAPID) method described in this document to voluntarily identify itself to a push service. The "vapid" authentication scheme allows a client to include its identity in a signed token with requests that it makes. The signature can be used by the push service to attribute requests that are made by the same application server to a single entity. The identification information can allow the operator of a push service to contact the operator of the application server. The signature can be used to restrict the use of a push message subscription to a single application server