The discussion the event concerns, if any.
The payload's main model, from the store. For post events this is the discussion.
As broadcast, e.g. Flarum\\Post\\Event\\Posted, revisedEvent, notification.
The JSON:API document the event carried.
For post events (new replies, edits, likes), the post itself.
One event from flarum/realtime, after Deck has put its payload in the store.