When you select Scan QR Code from your in-game menu, you can register Pokémon as Seen in your Pokédex.
Players can go to their registered Pokédex Entries, to share QR codes for other players to scan.
However, for certain Pokémon, when you try to generate a code in-game, it'll give the "This is a special Pokémon. There is no QR Code." message (as seen below).
This album hold such entries. Their QR codes were scanned from Japanese Ga-olé disks, which was marketed as being able to be scanned into SM and USUM. However, these QR codes are region locked to JP consoles only.