<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by AC*SE:
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 </font>
So your definition of nesting is the same as the IT definition.
The back-to-the-point-of-origin scenario is one for which a need comes up quite often (at least for me) in "real life." Here's the situation: I'm on a vacation - perhaps for 3 weeks - and the need arises for me to return to the point of origin for a short time - perhaps one day (a business emergency). So I buy a return ticket back to the point of origin and, upon return from that trip, resume my vacation.
So that's nesting
and back-to-back. Possibly, depending on the fares, it might be considered illegal. (??) But I can't imagine a carrier objecting to this; that would be beyond unreasonable.