Wow, like @bghazzal , I’ve spent a 35 year career travelling the world and even more so post-retirement and have never come across this. Incredible.

10 hours ago, bghazzal said:

Thanks - it's pretty wild though.

So basically, in our case, our trip was in IATA Tariff Area 3, but crossed 2 sub-tariff areas, first between Japan and Koera area and South-East Asia area (Narita-KL), then between South-East Asia area and the South Asian Subcontinent are (KL-Male).

In this situation, Rule Number 2 applies, i.e. the first carrier to cross the sub-tariff area is the Most Significant Carrier. This is JAL, on which the flight was booked, crossing from Japan and Korea tariff area to South-East Asia tariff area on the Narita-KL outbound leg.

However, and this is where it seems particularly perverse, if I understand correctly, this Most Significant Carrier rule doesn't apply to the whole flight, but to each check-in, thus resetting on the return leg!

So in our case, even though JAL was our Most Significant Carrier as per Rule 2, this did not carry over to the return flight, which resets the calculation at check-in.
On the return leg, the first carrier to cross a sub-tariff area was Malaysian Airlines, between the South Asian Subcontinent and South-East Asia tariff areas (Male-KL), thus becoming our Most Significant Carrier...

Wild. I've been air-travelling since I was in my mother's womb and have lived/worked in many different areas of the world, but I've never encountered a situation where the luggage allowance was reduced on the return leg of a single codeshare flight bought on a national carrier like JAL.

It's good to know about the Most Significant Carrier check-in reset rule, i.e. that the Most Significant Carrier is determined at check-in, thus resetting on the return leg.
If confirmed, this is really something to look out for, especially since you usually return with a similar amount of luggage on most return trips 😆
I guess it's not a super common situation, but definitely worth looking out for on codeshare flights within the same tariff zone...

Best I correct some of your terminology -

A Ticket is the overall booking - outbound and return legs, or round the world or whatever - it will have a six character booking reference on it (also known as a PNR)

A Leg is all or part of a journey from a Checkin to the point where you first collect your bags after that Checkin - so in your case Narita-KL-Male was a Leg, and Male-KL-Narita was the return leg

A Sector can be one flight or possibly several oneway flights with the same flight number - I will use Qantas as an example - QF1 is Sydney-Singapore-London Heathrow - all on the same QF1 flight number - basically you stop in Singapore and get off the plane for around 90 minutes while they refuel it and load new food etc. Coming from Melbourne I think it is QF37 (from memory) to Singapore and then I jump on to QF1 in Singapore to go on to London - so that is two sectors - but one Leg.

Another important thing to know is which airline is responsible when things go wrong. Say for instance on your return journey the Male-KL sector flight on Malaysian but with your codeshare JAL flight number had been delayed and you missed your JAL connection from KL to Narita - which airline is responsible for finding you a new flight from KL to Narita? The answer is Malaysian is responsible as they were the operating carrier for the sector that was delayed that caused you to miss your connection in KL on to Narita. Malaysians options would be to either put you on one of their flights to Narita (assuming that they go to Narita and not Haneda) or they could purchase a seat for you on JAL at a cheap rate deal between airlines for these sorts of situations.

If you have high frequent flyer status then you could also call the call centre for JAL and they would sort it out for you as you were on a JAL ticket. I was in this sort of situation a couple of years back - I was on a Qantas ticket and had completed the outbound legs from Australia to Cairo, and was due to fly home Cairo-Dubai-Singapore on Emirates with a connection in Singapore onto Qantas back to Australia. Unfortunately there was a massive flood in Dubai and the airport was trashed - no flights in or out of Dubai for several days as the airport was inoperable so I was stuck in Cairo. I learned of this while on my way to Cairo to catch the flight to Dubai. I tried to contact the Emirates office in Dubai but had no luck, the Emirates international call centre was swamped, so I rang the Qantas call centre and they were able to find a seat for me on some Emirates flights Cairo-Dubai-Melbourne a few days later. After I arrived home I submitted a claim to Emirates through their website for the costs for hotel accomodation in Cairo - a rather unnecessarily tedious process but eventually about five months later they paid, and I applied to Qantas for what is called Original Routing Credit - which got me the Frequent Flyer Status Credits and miles for the route that I had been originally booked to fly (Cairo-Dubai-Singapore-Melbourne) - that was quick and easy.

3 hours ago, TimG said:

Wow, like @bghazzal , I’ve spent a 35 year career travelling the world and even more so post-retirement and have never come across this. Incredible.

Its one of those things that people who fly a ridiculous amount learn off frequent flyer forums, and anyone else finds out the hard way at the wrong time - or most people never get affected by it and remain happily oblivious. Like you I fell into the flying internationally very frequently for work category and so spent a fair bit of time on frequent flyer forums learning how to maximise the benefit and also make each trip a less miserable experience. In my case I was flying from one continent to head office on another continent every fortnight for a 1 day meeting - do that every fortnight for several years in business class and you rack up decent frequent flyer status!

For anyone looking to go the @bvanant route with a reasonably priced backpack and then add padded dividers to suit, this bag might be of interest - https://www.temu.com/au/-2-in-1-detachable-carry-on-backpack-flight-approved-splits-into-two-bags-for-travel-56l-laptop-backpack-fits-17-inch-laptop-g-605520660375912.html

Splits into two bags - one for the camera gear, and a second one for under the seat to carry laptop etc.