Why is the normal Marvel universe called 616?
In Alan Moore’s Marvel UK Captain Britain serial, Captain Britain (and the Earth/Universe he was designated to protect) were assigned the numeric designation 616 by Roma. This was brought into Excalibur’s Cross-Time Caper by Chris Claremont and Alan Davis, and has subsequently been used by online comic fans as shorthand for talking about the “real” Marvel Universe, as opposed to alternate timelines or incarnations of characters from other media.