Why do some flight numbers contain a letter suffix?

Operational flight number suffixes (example: 353R or 928W) usually indicate that the flight is an unscheduled, ad hoc flight from an amended flight plan, such as air taxi, cargo, or otherwise not regular passenger flights. There are other reasons for letter suffixes, such as when a flight is delayed past midnight and the suffix serves to distinguish the delayed flight from the next day's regular scheduled flight. You should never see a suffixed callsign on a regular passenger flight record, or any flight created from the carrier's published schedule. We currently do not track the reason why a suffix was created by an operational data source.