Every Pokémon available in the Alolan Pokédex has it's own QR code - you can access it yourself if you've caught the Pokémon by heading to it's page in the Pokédex and pressing X - and scanning a Pokémon's QR Code with the QR Scanner will add that Pokémon to your Pokédex as 'seen'. The QR Scanner uses your 3DS's camera functionality to scan an actual, real-world QR Code.
They are different, albeit related, functions, so we'll explain them separately here: Originally introduced in regular Pokémon Sun and Moon, Island Scan and QR codes are a simple way to acquire Pokémon in-game (especially if you're prone to Slaking, geddit?). How QR codes work in Pokémon Ultra Sun and Ultra Moon There are a few caveats to the system - and it will take you some time to scan then all - but below we explain everything you need to know about Island Scan and Gen 7's QR code functionality. Ultra Sun and Ultra Moon's Island Scan feature works similarly, unlocking after you use the regular QR Scanner a certain number of times, only this one allows you to find and actually catch Pokémon in-game. Pokémon Ultra Sun and Ultra Moon QR codes are by far the easiest way to fill out the 'seen' aspect of your Alolan Pokédex, granting you the ability to simply scan a code - we have a full list below - and register that Pokémon as 'seen' there and then.
