Concerning skipped flights: when 2 completely unrelated flights are booked in 1 booking. Let's say San Fransisco to Miami with United on 1 February and a local flight from Ho Chi Minhstad to Hanoi with Vietnam Airlines on 1 July.
I'm almost certain that the Vietnam flight will NOT get cancelled when I didn't board the United flight in the US. Even if it would be "connecting" flights (meaning the departure of the second flight is close the arrival of the first one) it's not always the case, and certainly not when there is no relation between the two airlines.
The question is: what are the exact conditions that have to apply that could result in cancellation of all subsequent flights when a flight is skipped?
The second flight would most likely be cancelled despite it being different airlines on different continents flying months apart. This is part of the difficulty of fuel dumping as the first flight would be termed a "1X" and ones that are convenient to fly are difficult to come by.