I doubt there is a “usual”. It can stand for any name with those initials whether it’s a double barrel first name, or a first and middle name, or a first and last name. Honestly, the only DJ I’ve ever heard of personally is the DJ on the TV show Full House and as you probably know, it was a girl named Donna Jo.
I don’t know if there is a “general” but on Full House it’s Donna Jo I think
My cousins name is Daniel James, we’ve called him DJ all our lives.
I’ve known a girl, DJ, named Donna Jo. Both were always referred to as DJ.
Any name that has a first name starting with D and the middle name or surname starting with J.
My friend has a little boy called Jacoby Cameron, they call him JC. My friend from college is called Johnathan and his surname stats with D, we call him JD
Daniel Jacob or Jonathon I’d say
Either the persons initials or their first initial and jr.
It normally stands for digital jukebox lol …. i think