In the days when CityFlyer was a BA franchise they were allocated flight numbers in the 8000-8199 sequence, 80XX or 81XX being the common denominator. Each destination would have its own identifier and that would be the 3rd number. The last number was whichever flight of the day it was. For example DUB was 8080-8089, NCL was 8070-8079. The callsign was also linked to the flight number and destination, ie BA8081 would have callsign Flyer 81DA, 81 for flight number BA8081, D for route DUB and A indicating it was outbound. Got confusing when you had more than one destination starting with same letter. Dusseldorf used F as the route identifier.
Some flights were designated as through flights, i.e. LBA-LGW-DUS so the LGW-DUS leg would be flown as a LBA flight number. Could be that ScotAirways are operating Dundee-City-Somewhere else on the same principle?