Stand for & short for

UN is the abbreviation ...

I don't think there's a real strong rule here, but there is a difference between a diminutive and an abbreviation.

"kilo" ( diminutive) is short for kilometer
km (abbreviation) stands for kilometer.

Rick does not "stand for" Richard, it is "short for" Richard.

UN stands for. If you say it's short for, that means it's an abbreviation. For ex: apt. is short for apartment. NASA stands for National Aeronautical...

You can say or write either. In more formal writing situations you might use abbreviation.

UN is the short form for United Nations.

UN stands for United Nations.