Q) What constitutes a segment for the purposes of the fare rules?
A) A segment is a flight with a single flight number between two cities - whether or not it stops between origin and destination, and whether or not there is a change of a/c along the way

E.g. SYD-JFK is one segment (and does not count towards one transcon in NA) even though it stops in LAX

BUT

NRT-BKK is two segments - CX NRT-HKG and CX HKG-BKK as no single flight number on OW airline can get you between NRT and BKK