You can only scan 10 QR codes a day and you only get one attempt to catch that Pokemon, so be careful not to knock it out.
This basically means you can catch Pokemon that aren’t found in Alola normally in the wild including classic starter Pokemon like Charmander, Squirtle, Bulbasaur and so on, though it’s limited. Running this scans the island you’re currently on and finds/spawns a rare Pokemon that usually can’t be found in Alola – in real terms making it spawn on that island. The QR code system in Pokemon Ultra Sun & Moon does have one other major advantage: every ten codes you scan you’ll be able to run an ‘Island Scan’. Pokemon Ultra Sun & Ultra Moon Tips – How To Ash’s QR Code Pikachu
