I read:
If you're flying AAA-BBB-CCC and then CCC-BBB-AAA, you might expect to have four coupons: AAA-BBB, BBB-CCC, CCC-BBB, and BBB-AAA. A coupon doesn't necessary correspond to a segment, however. – choster Jun 4 at 19:18
In what case doesn't a coupon correspond to a segment?
A coupon is a discrete component of a ticket that is separately valued and separately redeemed. It's the record that you've paid for a portion of the trip, and the record as to whether you've used it or not. An airline segment by the IATA definition is a passenger enplaning and deplaning, though this term is used more loosely by lay people, and gets confusing in the GDS world where you can have "hotel segments" and so forth.
An itinerary can have gaps or "land segments/surface segments" as lambshaanxy notes, as on an open jaw or RTW ticket. The airline is not transporting you between those points, nor any other carrier on the ticket. They count as segments, but there would not be an associated coupon.
Most "direct" flights nowadays are indistinguishable from connecting flights, but it may be possible that you have a single coupon for multiple segments as well.
Just to add a source, an explanation on why indicating a land segment is used in airline reservation systems, and an example to choster's great answer, from the Wikipedia page on open-jaw tickets:
The open gaps between the cities show on the itinerary as ARNK, the same code that shows on an airline or agency's [Global Distribution System]. The term (pronounced arunk) means "arrival unknown".
The reason for this is that airline reservation systems (and major GDSs) require the segments following on sequentially, so arriving at one city, and then departing from another, will cause the system to return an error message. The ARNK field tells the system that this is intentional, and also allows for the ticketing system to blank the unused coupons of a ticket.
A typical destination open-jaw would look like this on an itinerary, with the ARNK on the second segment to show that it is not being flown on that ticket.
Segment 1: 11-NOV: SFO/IAD (San Francisco to Washington-Dulles)Segment 2: ARNK: Arrival Unknown or Surface Transportation from IAD to PHLSegment 3: 15-NOV: PHL/SFO (Philadelphia to San Francisco)
Also, here are the IATA definitions:
| Term Name | Description | Source of Description |
|---|---|---|
| Coupon | The portion of the Passenger Ticket and Baggage Check or Excess Baggage Ticket that indicates particular places between which the coupon is good for carriage. | |
| Segment | A leg or a group of legs from boarding point of a passenger to a deplaning point on a given flight. | IATA,PSCRMe34,CHPT 1,RESO 766 |
| Segment | (Sometimes referred to as City Pair) The operation between board point and any subsequent off point within the same flight. | IATA,SSIMe38 (2011),CHPT 1.1,RP 1761b |
FYI:
