Its a method used by most roleplay hosts, to advertise their roleplay. For instance someone in the chat says "abc to be a younger sister", if you were to join, you would reply with "abc". Replying with "abc" lets the host see if you would like to join, in a brief, convenient way. If you were to advertise your own rp (short for roleplay), but there's already another host advertising with "abc", you could use "123", "xyz", etc.
