To be more accurate, end-on-end fare construction allows you to build more than one itinerary into a single ticket. Since there is only one series of coupons, there is no nesting.

Nesting is the practice of flying an itinerary after completing the outbound journey, but before completing the inbound journey of a different ticket.

If the nested itinerary takes you back to the point of origin (or a co-terminal) of the first ticket, then that is considered back-to-back. If you go to a third city, then