There is generally only one rule that applies for baggage when you have connecting itineraries (aside from trips to/from Canada/USA more on that in a minute), which is the most significant carrier rule. To summarize the rule, only one airline's baggage policy will apply for the whole trip from the origin to the final destination. That airline will be the first airline which:
- Crosses an IATA zone (basically IATA splits the world up into three major travel zones: zone 1: North and South America, zone 2: Asia-Pacific, and zone 3: Europe, Middle East and Africa)
- Crosses an IATA subzone (these are regions within an IATA zone so for instance North to South America or Canada to the USA)
- Crosses an international border
Note: this list is numbered meaning the itinerary is evaluated by first looking to see if any flights match the first rule, then if nothing matches the first rule the second rule is considered, if nothing is matched there then the third rule is considered. Equally important, it is the carrier that operates the flight and not the marketing carrier which the rule applies. So if you fly an British Airways operated flight between Sydney and London, it will be British Airway's baggage rules not Qantas' baggage rules that apply even if you are flying on a Qantas flight number.
The major exception to this rule, which again applies virtually everywhere is flights to/from Canada/USA. The rule there is simpler (and maybe fairer?) the first airline which appears on the ticket will decide what baggage rules to apply. Confusingly they can either apply their own or use the most significant carrier rule outlined above.
Some worked examples for the peanut gallery:
Sydney to Darwin (Qantas) -> Darwin to Singapore (Qantas) -> Singapore to Helsinki (FinnAir)
In this case the baggage rule that would apply is FinnAir's because FinnAir was the first airline on that ticket to cross an IATA zone (from zone 3: Asia Pacific to zone 2: Europe).
Sydney to Melbourne (Qantas) -> Melbourne to Dallas (American Airlines)
In this case Qantas' baggage rules apply even though American Airlines is the most significant carrier in the journey as we are dealing with the first carrier rule as it is a trip to the USA, and Qantas will apply its baggage rules and doesn't apply MSC in these circumstances per their website.
Mexico City to São Paulo (Aeromexico) > São Paulo to Buenos Aires (Aerolineas).
In this case Aeromexico's rules would apply for the whole trip as it was the first airline to cross an IATA sub zone.
Frankfurt to Munich (Lufthansa) > Munich to Lisbon (TAP)
In this case TAP's baggage rules would apply since it is the first flight crossing an international border.
-RooFlyer88