Think of it as an extremely sparsely documented Extra.
So, here's the reference example. I'll probably be pointing back to it now and again.
The campaign is a sendup of kids' TV. Pretty much everyone's playing a kid on a bike but one player decided they'd play the local superhero: Artie, the Strongest Man in the World. (Because this is a sendup of kids' TV Artie hasn't so much as thrown a punch.)
During a climactic scene, the GM puts a challenge in front of the players in order to get to the Powerline concert on time. As the first roll of this challenge, Artie will uproot the gate to McKimson Park (brick and wrought iron, two stories high) so everyone else can cut through it on their bikes.
The GM sets the difficulty for Artie's roll at +5.
That +5 doesn't have to say anything about the difficulty any other citizen of Daniels-Oldfield County might have in uprooting the gate to McKimson Park, or even the difficulty Artie might have in doing so any time other than right now. It just has to reflect the stakes and the drama of the scene in the moment, and no one other than Artie needs to be capable of it.
But how did they decide that in the first place?
Extras and Documentation
Well, pretty much the same way everyone decided Zird should just be able to do magic things in the Extras chapter, really.
- Does the extra influence the story, and if so, how?
- Does the extra let you do things that no other skill lets you do?
- Does the extra make your existing skills more useful or powerful?
- How would you describe the use of the extra?
- If the extra influences the story, then it should use aspects.
- If the extra creates a new context for action, then it should use skills.
- If the extra makes skills more awesome, then it should use stunts.
- If the extra can suffer harm or be used up somehow, then it should take stress and consequences.
A permission is the narrative justification that allows you to take an extra in the first place. For the most part, you establish permission to take an extra with one of your character’s aspects, which describes what makes your character qualified or able to have it.
-- Fate SRD, "Creating Extras"
So the reference example is a game about kids' TV characters. Physical conflict is going to be rare. Most conflicts will be mental or social, or maybe some kind of wrapper that's meant to model an exploration. Being the Strongest Man in the World isn't going to be especially helpful.
But at the same time, kids on bikes can get up to a lot more mischief if the Strongest Man in the World is there to help.
So how they all decide to approach Artie's "extra" is that Artie doesn't have to roll to do anything that might be a challenge of strength or endurance for a regular kid on a bike, and can roll to perform feats of amazing strength that no kid on a bike has a hope of doing.
When everybody takes a break from the campaign to play the megacrossover with the other network shows, where all their hero types have to fight an alien overlord, Artie doesn't get any more detail on the extra for kind of the opposite reason - it's not really an extra at all in this company. Everybody's got some kind of superhuman ability and stands on a fairly equal footing.
And in games with more fixed skills like Save Game or Fate Accelerated, this aspect of Extras -- the "permission" granted by Aspects -- can fairly readily mutate those skills, such that the Vice-Dean of Mechanomancy can Cleverly Create an Advantage and what comes out is golems wreathed in blue fire that no other PC could make.
All this can happen because
Except in relation to other numbers, right? That's just maths.
So how much can a character with Great (+4) Physique bench press? How much can a character with Fair (+2) Resources spend before going broke?
The truth is, it is unknown and the Fate system is reluctant to pursue a specific answer.
[...] Remember, a skill roll is a narrative tool, meant to answer the following question: "Can I solve X problem using Y means, right now?"
-- Fate SRD, "Skills and Specific Measurements"
Artie is going to help meet the challenge by doing something that needs his strength, because he's the Strongest Man in the World. In our example, that's lifting an imposing obstacle out of the way - the gate to McKimson Park. The difficulty to uproot that gate is the difficulty for Artie to dramatically contribute to a pivotal scene in the story. The number doesn't relate to any number like shear tolerance or total mass that a civil engineer might know about the construction of the gate. It only relates to the progression of the story.
What "Aspect Permission" Means, and How to Adjudicate It
"Aspect Permission", then, is an Extra that isn't really written down on character sheets or balanced based on anything but understanding - a different way things work for one character that isn't a concern for balance, for one reason or another. And something unique like that is usually related to one of their Aspects.
Now, because it's not really written down, and certainly everything that might be possible isn't written down, it's possible for disagreements about the exact scope of an extra to come up in play. How you handle it depends on how much of a surprise it is to you, and whether or not you can come up with a good roll, with interesting consequences for success and failure.
That makes sense, come to think of it.
"Alright, Artie, that's everyone over the ramp you held up for them. Now how were you planning to get over Antigarry River?"
"Can't I just take a running jump? Surely, the Strongest Man in the World can throw himself!"
No good roll? Don't charge. "Right, sure. So how would they have shot this in the show?"
A good roll? Don't make it a hard one. "Right, sure. But how's your aim? There's the pine forest on the other side, after all. Overcome a +2."
I wasn't expecting that.
"The large shaved man in the suit folds his arms. You infer from context this means Mayor Pat won't be taking any visitors today - at least not through the front door."
"Wait! I'm the Strongest Man in the World! Surely I've done enough for this city that Mayor Pat would be willing to listen to me. And my friends!"
No roll? Charge a Fate Point. "Hmm. Well, if you're willing to spend a Fate Point, tell me what you do to convince this guy that you're the Artie everyone's been talking about."
A good roll? This one can be a bit hard and "expect" a Fate Point. "Sure, roll Rapport against this guy's suspicion, but suspicion is kind of his job. You can spend on Strongest Man in the World if you come up short."
I hadn't even considered that.
"Ah, tough break. Looks like the key to that odd box in the attic is now well on its way out to sea."
"Not to worry, friends! Surely one of my chums in the Civic-Minded Six, which I have coincidentally never mentioned until right now, would be glad to help the Strongest Man in the World!"
No roll? No dice. "That's... a bit of an ask. If you really want that to be a thing, we can take a break and hash out how it might work?"
Somehow you have a good roll? Charge a Fate Point up front to simulate the stunt this surely is. "...hmm. If you're serious, we'll talk about this later, but for now give me a Fate Point and make a Contacts roll. Sockeye Salmon Woman doesn't forget a favor but she's the best one for the job, no question."
Note that if you, the GM, think that an activity will be in the scope of someone's extra, and you plan a scene around it, you should never charge someone a Fate Point to be in your scene the way you want them to be.