In Starcraft II, your “online handle” comes in two parts: your name, which you can choose, and your “dot identifier”, which is assigned to you.

You can’t friend me unless you know my .identifier (or unless you’ve just played a game with me). So I always write judy.933 so that people can add me.

Blizzard did this as an answer to name collisions, so that there could be multiple “LionShadowWarriors” in the world. In other games, people have problems when they lose their name (like if their account is compromised or if a pro gamer has to change their name when they change teams). The .identifier system fixes this, but introduces other problems (like having to write down your .identifier), and also a lot of trolls take the names of famous pro players just to mess with people.