Its an agreement between you the taxi driver and the marshall (at the rank) but mainly the taxi driver agreeing to take you somewhere "local" which is normally not covered in the zonal agreement.

The marshall can ask the driver but the driver can also refuse.

The main part of the deal is that if they do agree to take a short journey, the marshall will give them a "queue jump" ticket so when they come back to the airport after that job they wont have to go to the back of the "very long" taxi queue!