Mercure's spec has a bit more meat on its bones in terms of providing an adequate description of the protocol.

From https://mercure.rocks/spec :

Mercure provides a common publish-subscribe mechanism for public and private web resources. Mercure enables the pushing of any web content to web browsers and other clients in a fast, reliable and battery-efficient way. It is especially useful for publishing real-time updates of resources served through sites and web APIs to web and mobile apps.

Subscription requests are relayed through hubs, which validate and verify the request. When new or updated content becomes available, hubs check if subscribers are authorized to receive it then distribute it.