Most international air cargo is carried to its destination in the holds of passenger aircraft. The amount of cargo uplifted on any aircraft is subject to the passenger and baggage load, the type of aircraft itself and the distance of the flight sector involved. Dedicated cargo flights also operate on key global routes.
Airlines accept airfreight limited to an airport-to-airport contract in accordance with the Warsaw and Montreal Conventions. These define the legal, contractual and liability (for loss and damage) obligations of the airlines as well as those of the shipper.
Once cargo is pre-processed - known as being ready for carriage - by the freight forwarder, it's delivered to the airline's cargo terminal at the airport of departure. The cargo terminal operator's primary responsibility is to the airline - ensuring that all cargo for a given flight is transported to the aircraft's side at the appropriate time, ready for loading.
With few exceptions, the airline's cargo terminal operations are sub-contracted out to third-party operators contracted to the airline(s). The airlines themselves pre-plan their fuel, passenger, baggage and cargo loadings well in advance of departure. In the event of last-minute, unforeseen circumstances, cargo is sometimes offloaded and held for a later flight.
Although airlines do allocate flight details in advance for cargo movements, under their Conditions of Carriage they can't be bound to them and held responsible for delays in consignments arriving at their destination.