Alert & callout
An alert reports a live condition of the data; a callout is the author speaking. Data confidence is a message, not an error.
Alert, banner or toast
Alert
A condition of the DATA, right now. Lives inline, near the thing it is about.
Banner
A condition of the whole VIEW, which stays until it changes.
Toast
A confirmation of something the user asked for. Transient.
Alerts
They state what is true, what it means, and what to do — in that order.
Information: Zoning shown is the current planning scheme
Caution: Data is older than we'd like
Error: We couldn't reach the council register
Success: Report emailed
The tone is announced in words as well as colour — a screen-reader user gets "Error" the same way a sighted one gets red. The body stays neutral ink in every tone: tinting the prose as well as the heading makes a caution message harder to read than a plain one, which is backwards.
Callouts
Editorial asides for docs and articles. An alert reports a live condition; a callout is the author speaking — so a callout never carries actions, and it is never role=alert, because nothing has happened.
A callout's colour is generic status, never planning meaning. Caution is the only status hue it may take.