Back-half
The part of a short link after the domain: the key the server looks up to find the destination.
It can be generated, which produces something like /aB3x9, or chosen, which produces something like /spring. A chosen back-half is worth the effort on anything a person will read, because /spring survives being retyped from a poster and /aB3x9 does not.
Case usually matters. On most shorteners /Spring and /spring are two different links, because the lookup compares the stored key exactly.
Some back-halves cannot be used on a shortener's main domain, because a real page already answers at that path. On your own domain, nothing is reserved.