Yes , it is very complex topic , but to put it simple

usually in a booking you can have each segments with different fare , therefore each have his own rule , and it can happen that the whole booking might be restricted because of one single segment .

if you do the following trip = seattle - atlanta - mexico - sao paulo , and the mexico-sao paulo flight is operated by aerromexico in basic economy (which means no free luggage)

Even if you are allowed a free luggage on the delta flights from seattle to mexico , then you will most probably have to pay your luggage for the entire journey , just because one flight is more restrictive than the others

That is the reason why airlines often say = "Most restrictive rule applies when combined two or more fares together inside one booking"