A risk scenario is the unit of analysis that everything quantitative downstream depends on. The shape of the scenario decides what threat actor you are reasoning about, what asset is at stake, what the loss looks like, and which controls are relevant to the answer. "Cyber risk" is not a risk scenario; "customer PII exfiltrated via a compromised marketing-automation administrator account" is.
Useful scenarios share three properties. They name a specific threat community or actor type, because the frequency of the bad thing is shaped by who is doing it. They name a specific asset and the loss form, because the magnitude is shaped by what happens after the event (incident response cost, regulatory fines, customer churn, contractual penalties). And they are written at a level of granularity where each leaf of the FAIR decomposition tree has a defensible estimate behind it. Scenarios that are too broad collapse into the heatmap; scenarios that are too narrow proliferate into a register no one can navigate.
The discipline pays off twice. During analysis, well-shaped scenarios produce numbers an executive can reason with rather than colour codes that get argued about. During incident response, the scenario library doubles as a runbook index: when the real event maps to a scenario already in the register, the response can lean on existing decisions rather than be improvised. The Risk Investigation Agent maintains the scenario catalogue as part of the risk register, so the analysis and the operating record stay in sync.