My convoys represent the transfer of cash from one place to another, they don’t generate extra income. Hence, each one is a link in a chain of land - to - sea(convoy) - to land, with all 3 (or more) links needing to be held for the transfer to take place, along with a demonstrable route to the desired IC destination.
They are either lend-lease convoys to send money to an ally, or colonial convoys needed to transfer money from overseas possessions. It is absurd that the UK can collect cash for India when the home island is surrounded by enemy ships.
There are a couple of overland routes also, i.e. Persian corridor.