The engines ECU map

Ford created 2 in total - AE was the map the car left the factory with - AF was the map Ford created to resolve some issues with AE map such as cold start issues & other small things - most cars on AE had the AF map loaded when they went into Fords for a service, diagnostic, etc

Problem with AF map though is the more heavily you mod the car, it can develop a misfire around 4.5k revs