Every user on smash.gg is assigned a unique, 8-character code known as a discriminator.
You can find a user's discriminator listed on their User Profile.
It also appears in the User Profile URL.
You can also pull up the discriminator for every user entered in one of your tournaments using the Attendee Export.
Discriminators can be useful when trying to search for a user with a commonly-used gamertag. You can plug the discriminator into the search field when you are adding an attendee, adding an admin, or blocking a player to pull up that specific user.