End-on-End Ticketing - Definition?
 

Say I want to fly KUL-LHR-ARN on BA, returning a week later. Then I buy a separate ticket, also from BA for ARN-LHR-LAX and return, nested inside the other itinerary. I will not be combining the tickets. Even though the tickets aren't combined, are they considered end-to-end (not allowed), or simply a side trip (allowed).

I'm posting the question here because it seems generic within IATA.

There is no issue with any kind of nesting with BA.

It is not end-on-end. That's a specific construct of fares where A-B + B-C are sold as a single itinerary with two fare components. It is also not a side trip as you are buying separate tickets.

The main thing to look as is if you are avoiding a minimum stay rule with the bookings, especially if returning to the point of origin. In your case the nested "return" is to LAX, not KUL, so no issues there.

This is a broad statement and not one I'm sure stands up to review.

Last time this discussion was had on the BAEC forum there were no examples of nested flights ever having been an issue with BA - this included 50+ people totaling hundreds if not thousands of tickets. Does anyone have any evidence to suggest that there are examples which are problematic?