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.