A coupon is either a flight segment (booked as single flight number) or a surface segment (VOID on paper tickets).

Note that a single coupon may in fact cover more than one flight - eg NZ1 booked as LHR-AKL is actually LHR-LAX-AKL with a short transit in LAX. NZ1 could also be booked as 2 coupons, LHR-LAX and LAX-AKL. This might be due to having a stopover in LAX (1 or more days), booking into a different class on the 2 flights, or for other reasons.

The xRWSTARx rules used to specify 24 flight segments. When they changed to coupons it meant surface segments got counted in the limit whereas previously they did not count. xONEx (the One World Explorer RTW) has recently had a similar change to include surface segments, but with a maximum of 20 rather than 24.