A bit of info I found interesting

CLOSED LOOP –
Closed Loop is a term in which the PCM uses feedback from the O2 sensor to make corrections to the air fuel ratio. Another name for this term is short term fuel trim. The PCM makes immediate but temporary corrections to the fuel delivery to maintain the AFR at 14.7. The only ratio that can be maintained in closed loop is 14.7. This is due to the nature of the type oxygen sensor used on most passenger cars. There are other types of O2 sensors called wide band sensors, but they are expensive. They can be used to monitor the AFR at other than 14.7 AFR. The short term fuel trim value is called the Integrator. The value of the integrator varies above and below 0 with 0 being no correction. For ex., if the Integrator is +5, the PCM is adding fuel because the O2 sensor is reading a lean mixture. If the Integrator is -6, the PCM is removing fuel because the O2 sensor is reading a rich mixture. Anytime the system is not in closed loop, the Integrator will immediately return to a value of 0 and stay there. There is only one Integrator and its value is solely dependent on the O2 sensor. When the engine is started, the PCM will keep the Integrator at 0 until the PCM determines that the O2 sensor is working correctly and that the engine temperature and time delay constraints before entering closed loop have been satisfied. Once the PCM goes into closed loop, the Integrator begins to adjust the fuel delivery to maintain a 14.7 A/F ratio, however, the Integrator term is only weighted half as much as the Block Learn term (LTFT). The Integrator and BLOCK LEARN work together to re-tune the system to match any engine'* characteristics, up to a practical limit.

nice write up