Island reserve has been mentioned.
The VC10 was, often, unable to divert from Bermuda to a mainland US airport, on a flight from the UK, so we took island reserve.
If I remember correctly, this allowed for 21/2 hours holding overhead Bermuda at 10000? feet (20 tonnes?).
It was for the blocked runway situation, although, I seem to recall, there was a "cross" runway in those days which was usable with sufficient wind.
The aircraft set off, a "last point to turn" was calculated, at which a decision was made to continue if a landing was assured or to divert to the US mainland.
This could be before top of descent.
Others may be along to correct my memory.
Another option was a tech. stop in Shannon, or the Azores, although I never did that.