On-the-fly functions and modes, are missions generated in the field using the mobile app and the drone position as inputs.
It is possible to create and customize on-the-fly functions for a variety of use cases (such as cell tower inspection, crash scene mapping, custom panos, roof inspections, wind turbines, etc).
An Enterprise account is required.
For Basics about on-the-fly functions, see this getting started tutorial about Waypoints and Follow Mode.
Existing on-the-fly functions and the code powering them can be explored here.
The following Documented Waypoints On-the-Fly Function is a great starting point to understand how functions work, and how the Dronelink SDK is used to create missions.