Rarity is not how often you see one. It is how many are left.
There are two different questions hiding inside "was that rare?", and separating them is most of the answer.
The first is how often you personally see one. That says more about where you stand than about the aircraft. Live under the approach to a hub dominated by one airline and its fleet will feel commonplace to you and exotic to someone two hundred kilometres away. Personal frequency is a fact about your garden.
The second is how many of that type still exist. That is a fact about the world, it is the same for everyone, and it is the one worth building a collection on. An aircraft you see weekly might still be one of only a few hundred flying; a type you have never caught might have thousands of examples and simply not come your way.
Rarity done properly measures the second. It asks how many airframes of that type remain in the global fleet, and answers with a number rather than an impression.
Plane and Simple weighs every catch against a bundled database of 91,812 airframes. The distribution across types is wildly lopsided.
There are 6,924 Airbus A320s in that database. There are 809 Boeing 747s. There are 252 Airbus A380s. So an A380 is roughly twenty-seven times scarcer than an A320, and the 747, an aircraft most people would call iconic rather than rare, sits between them, having quietly become unusual while nobody was paying attention.
This is why a feeling is a poor guide. The A380 is enormous, unmistakable and famous, so catching one feels like catching something common; it is one of the scarcest widebodies you can see. Meanwhile the regional turboprop nobody photographs may exist in far smaller numbers than the jet parked next to it.
The long tail is where the hobby gets interesting. Below a few hundred airframes you are into types that are being retired, operated by a single carrier, or built in small numbers for a specific job. Those are the catches worth waiting for, and they pass overhead more often than you would guess.
A rarity figure is only as good as the counting behind it, and the naive way of counting produces nonsense.
Aircraft databases do not spell a type consistently. The same aeroplane appears as "Airbus A320-214", "Airbus A320 214" and "Airbus A320neo", and the manufacturer alone may be written as "Embraer", "Embraer S A" or "Embraer-empresa Brasileira De". Count the raw model strings and a bog-standard A320 comes back as one of a kind: the single most common airliner in the sky reported as a unicorn.
So the strings have to be collapsed to a family before anything is counted: corporate noise stripped out, manufacturer names normalised, variants folded into the type they belong to. Only then does the count match reality, which is how the figures above (6,924, 809, 252) come out right.
The failure mode this avoids is not cosmetic. A rarity system that quietly splits one aircraft across two spellings halves the apparent population of everything it touches, and then congratulates you on catching something ordinary. A collection you cannot trust is not a collection.
Some types are worth knowing about before they pass, because you get one chance to recognise them.
Four engines is the single most useful cue in the modern sky. Twin-engine aircraft have taken over almost completely, so four engines under the wings means something older or bigger: a 747, an A380, an A340. A decade ago this was unremarkable. It is now the clearest signal from the ground that something uncommon is overhead.
Freighters are worth catching because they outlive their passenger equivalents. Types retired from passenger service years ago carry on hauling cargo, often at night, often on routes that pass over nowhere in particular. A 747 overhead at three in the morning is far more likely to be carrying freight than people.
Anything with a propeller at commercial size. Regional turboprops exist in much smaller numbers than the jets they fly alongside, and they operate at lower altitudes where you can actually see them.
Registrations from far away. A tail registered on another continent, over your town, means either a long-haul route you did not know passed overhead or an aircraft doing something out of the ordinary. Both are worth logging.
Plane and Simple tells you the population of what you just caught, at the moment you catch it.
When the app calls a sighting uncommon it is not grading on a curve or guessing from how often you have seen one. It is reporting how many of that family remain in a database of 91,812 airframes. The claim is checkable, which is the point.
Two of the nineteen achievements are built directly on it. One of a Few is awarded for catching a type with fewer than forty airframes left. Unicorn, hidden and one of three that are, needs a type with fewer than ten. Neither can be planned for. You cannot go and find a unicorn; you can only be looking up when one goes over.
Which is, in the end, the whole hobby: the reward is for having looked, and the app's job is to make sure that when you did, it counted.
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