A 4-6 digit code is frequently used as a replacement to username and password to quickly return users into the application. It is usually referred to as pin.
I began to wonder whether the correct spelling is a "PIN" as it is an abbreviation of Personal Identification Number, or whether it is a reference to physical lock pins and therefore should be spelt as "pin" (https://en.wikipedia.org/wiki/Pin_tumbler_lock).
Turns out that it is a difficult topic to research and after spending half an hour I couldn't find a definitive answer on Google. The spelling that I find on different websites is very inconsistent.
Looks like the code used in ATM is referred to as Personal Identification Number which I find strange as I would've thought that Personal Identification Number should be static while the card access code could be changed by the user at any time.