Basic EFI systems have 2 modes, Open loop and Closed loop.
In closed loop the ECU will control fuel to 14.7:1 mix, this is used for idle and all low load applications.
Once you hit a set of defined figures, generally load and throttle position,it will then operate in open loop mode, and will follow a 2d or 3d graph of air fuel ratios for load and RPM, then give the fuels thats programmed for.
When you tune a car, you edit that OPEN loop mode as thats where the fuel can go rich or lean, and you can change it.
As I mentioned in another post they also have fuel trim setting where it will trim the fuel for different mode like idle, cruise and Wide Open Throttle, this is what gets learnt when you mod stuff and is in a soft RAM, when you reset the ECU you force those figures to reset and get re learnt.
They are generally around 15% +-