Under used flights
I recall many years ago when BA operated the proper shuttle and then super shuttle services from LHR to Man, BFS, Edinburgh and Glasgow, the guarantee was turn up and fly - no advance booking required, The BA TV advert talked about 2 pilots, 5 cabin crew and one very important passenger (on a B757).
I flew several times on the back up plane (sometimes a B737 or BAC1-11 but often a B757) with fewer than 20 passengers.
I know there are PSO type routes in the US and Canada but in Ireland I have flown Derry to Dublin in an ATR 42 with 5 passengers (and only 2 got off the inbound at Derry) and Carrickfinn to Dublin, also in an ATR 42 with 12 passengers