Most aftermarket ECUs and almost all factory ECUs have Closed loop fuel control. To obtain Closed loop fueling you must have an Oxygen sensor (O2 sensor) and an ECU able to read the voltage from it and adjust the fuel mixtures either + or -. O2 sensors come in many variants, common types ranging from 1 wire upto 5 wire and in narrow or wide band (annother blog).
Closed loop fuel control usually happens at idle and low engine speeds to achieve the optimum air fuel ratio (AFR) of 14.7. That is; 14.7 parts of oxygen to every 1 part of Unleaded petrol. Which gives a combination of the most efficient burning rate VS. optimum tourque.
Mixtures of 15.0 - 1 for more fuel efficentcy can be common on more advanced systems but a loss in tourque is usually a result, therfore resuming an AFR of 14.7 -1 when throttle is applied is typically required. incorrectly sized injectors and poor injector spray patern can impead a tuner from reaching this ratio in an aftermarket application.