friend is flying from London to NYC and they were told they might need to land in Toronto.
So my guess this is what it means "case by case". Some which are close to destination would be manually routed, others will land in different places.
Exactly. London to LAX may land in New York or the first available airport, or may route somewhere else.
Exactly. London to LAX may land in New York or the first available airport, or may route somewhere else.