Editing Unwritten: System Reference Document/Playing a Scene (section)

====Invoke aspects====

If the involved players or the GM are not satisfied with the roll, they can now invoke aspects (either using fate points or free invokes) for bonus or to re-roll the dice. Boosts can also be spent for bonuses at this time. You can invoke several different aspects using fate points, but each aspect can only be invoked once for this action by fate point. However, multiple free invokes can be used on the same aspect in this one action (on top of a fate point, if needed).

Players and the GM can go back and forth spending fate points, free invokes and boosts as much as they like. Once they are done, the outcome is figured out again and the action is resolved.