It's the option you hit when you can't find the item for the order (you can skip or INF -- skip means you delay it until later, INF means you basically remove it from the order). So you go to the backroom location that it's located at, but it's not there and it also isn't on the salesfloor -- you hit INF and move on to the next item.
Ideally, you'd try to find it, but there's only so much time you can waste digging through repacks or waving around an RFID gun. If it's a salesfloor location and it's empty, we'll check that area (neighboring sections and endcaps) before moving on. If it's a backroom location, we'll check nearby WACOs or maybe just quickly scan the aisle before moving on. In both cases, we'll also type the DPCI into MyWork and see if it's anywhere other than the location we were sent to. If all that fails, we INF (rather, we walkie the ETL-Log or LOD and inform them, then we INF).
INF is a tracked metric for each individual, as well as the store. The higher the INF score, the worse it appears you/the store is doing. TMs with a high INF score typically get scolded by their TL/ETL/LOD while stores with a high INF score get scolded by the DTL/GVP/GTL.
I imagine it exists for a reason -- a high INF score likely means a disorganized store, not a lazy TM. So a TM would hit INF if they can't find the item, then leadership would look into why the floor counts are off/the backroom locations aren't accurate.
But, of course, we "cheat" a bit and spend time looking for the item instead. It's like exiting out of a Move batch when you get to a location that doesn't have an item it should have, Audit it, and reenter the batch -- you're cheating the system. AIS exists to point out errors, not to be bypassed in order to keep everything in the green.