What is the difference between Username, Display Name and Callsign?

Username is only known to you, and is only relevant to logging into your account. Display Name is the public facing name with a 5 digit disambiguator string added automatically. (MyDisplayName#12345). Callsign is a digital collectable that entitles you to owning your unique name within Shrapnel. There can be many Joshua#12345, Joshua#58435, Joshua#73854 but there can only be one Joshua and that is through purchasing the Joshua Callsign - of which there is only ever one.