What is a sequential ticket?

A sequential ticket arises with round trips bought on a single ticket, or itineraries like Antalya–London via Istanbul. Under IATA rules, all flight segments on a sequential ticket (each flight counts as one segment) must be used in order. So if you have a round trip and can't make the outbound flight, contact whoever sold you the ticket — agency or airline — and try to split the PNR; only cancel the outbound leg after the PNR has been split.