Originally Posted by DTW-HomeyFour
I was "FIM'd" on a US flight a few months ago due to misconnect. I was on a DTW-GSP "direct" flight, which meant it stopped in PHL. We were delayed departing DTW due to weather, so we missed the connection to the 2nd half of our "direct" flight. The computer showed my ticket as being used, as it could not distinguish how we were stuck in PHL since we were "direct" to GSP. So, a FIM was issued, which appears to be like a paper ticket essentially. The CSR filled it out by hand, and stapled it to the boarding pass. It looked like it could also be used for an interline as well.
Interline is primarily what they are for. It substitutes for having to reiussue a ticket. Agents are always told to reissue if they can; FIM as a last option, though irregular operations tend to see lots of FIMs in order to process long lines. It's up to the agent to decide but usually the decision to use them or not is obvious. They generally are rewritten in full fare booking codes so the receiving carrier often gets more money than the passenger paid, penalizing the airline that had the customer in the first place.