welcome to ft. that is most probably a code share. i am not famaliar with delta code shares, but lh and ua do a lot of code sharing. the 3 digit number is usually the filght number of the airline doing the flying. the 4 number is that of the airline sharing the flight. those are not hard and fast rules. some airlines when codesharing use completely different numbers for the same flights. you can usually get a pretty good conficence by checking the flights to the city pairs on orbitz.