Imagine a letter sent from London to Sydney in 1935. The surface rate was low, but the airmail surcharge was exorbitant. Many senders couldn’t afford to pay the airmail fee for the entire journey. However, they could afford to pay for the letter to travel by air only as far as, say, Marseilles or Singapore. From there, the letter would revert to slow surface mail (ship or train).
In the world of aerophilately (the study of airmail), "Jusqu'à Airmail Markings" Jusqu-a Airmail Markings- A Study Ian McQueen
If you are fortunate enough to locate a copy of Jusqu’à Airmail Markings – A Study (original print runs were likely under 500 copies), or if you find a digital scan in a philatelic society library, here is how to use it: Imagine a letter sent from London to Sydney in 1935
When you obtain a copy, do not read it cover to cover. Use it as a dictionary. Keep it next to your magnifying glass. When you acquire a cover with a French instruction, look up the location first, then check the rate, then verify the die. However, they could afford to pay for the