Originally Posted by Palal

So all of this terminology goes back to the days of paper tickets. Others that remember better will probably correct me.

Each paper ticket had a bunch of coupons. Each coupon would be a leg. Each paper ticket could have a maximum of "n" coupons, and if you needed more than that you'd have to have multiple tickets with different ticket numbers issued.

"coupons must be used in order" means that you have to fly the segments in the order ticketed, or in other words - ticket book coupons must be torn off in the order that they're written.

In the era of e-tickets, there may still a limit to the number of segments/coupons you can have (primarily due to the antiquated systems that GDSes use), so that may require multiple eticket numbers, although I don't think I've seen this lately.

Any time your e-ticket has to be reissued, a new number gets assigned to it, but the PNR history still contains the old ticket number.

I'm guessing you know that a PNR is now like a ticket booklet, containing all the relevant info about the segments, their order, etc., as well as the ticket number. If multiple airlines are involved you may have multiple PNRs, one for each airline, and possibly one for the GDS.