The briefing is Nell's answer to "what do I need to know right now?" It ranks what arrived by who it's from and what it needs from you — people you actually correspond with outrank mailing lists every time — alongside today's calendar and open tasks.
It refreshes as your accounts sync and re-frames itself through the day (morning, afternoon, evening editions at times you choose in Settings). The ranking is deterministic and grounded in your own correspondence history: the AI phrases the briefing, but it can't invent or promote items.