Sandbob
For dispatch fuel planning you normally have to include alternate fuel. But sometimes you can plan without it, if it is less than 6 hours away, and weather is good, and they have two runways, etc, etc...
For inflight determination of fuel required at the decision point of a re-dispatch, you can also "benefit" from that. I mean, at departure you were at 11 hours from destination, but now, from the decision point, you are at only 2, so if the conditions are met (weather, independent runways, etc...) then alternate fuel is not required.
However, when alternate fuel is not required, an additional fuel reserve of 15 minutes is required.
hope it helps