Here “alt” is being used as an abbreviation of “alternate” - I am pretty sure it’s not proper English, just flows better if your going to have to use the word a lot.

I really like how the TV show Fringe named some of the alternate versions of the main characters - Walter and Walternate (short for Alternate Walter), Olivia and Bolivia (short for Olivia-B), etc. though many of the characters didn’t lend themselves to that naming or their alternate counterparts weren’t remarkably different.

Then Rick and Morty came along and there were so many alternate universes they just gave them all numbers - and added the twist that you never know which Rick from which universe your watching in any given episode which also conveniently explains away any continuity errors (sorry nit-pickers) and creative license needed for a given storyline.