[unclebucknasty]

I think it's more of an API to allow Server B to register its interest in receiving notifications from Server A.

Of course, Server A must "speak Rest Hook" in order to enable that, so in a manner of speaking I guess it is a way to configure Webhooks on Server A.

But, it basically exposes a common interface for Server A and Server B to establish Webhooks. Server A can tell Server B which notification subscriptions are available (i.e. for which events), and Server B can then choose to subscribe, providing the callback URL for the Webhook.