Editing Unwritten: System Reference Document/Game Basics (section)

===Declaring a Detail===
Sometimes, you want to add a detail that works to your character’s advantage in a scene. For example, you might use this to narrate a convenient coincidence, like retroactively having the right supplies for a certain job (“Of course I brought that along!”), showing up at a dramatically appropriate moment, or suggesting that you and the NPC you just met have mutual clients in common.

To do this, you’ll spend a fate point and justify your story details by relating them to your aspects. GMs, you have the right to veto any suggestions that seem out of scope or ask the player to revise them, especially if the rest of the group isn’t buying into it.