Kind of guessed that would be the answer. :smile:
I think its along the lines of this even though this is for a different ECU:
Q: What Is "Fuel Trim"?
A: "Fuel Trim" is the fine tune control of fuel delivery by the ECU. To accomplish this the ECU increases or
decreases fuel delivery by increasing or decreasing the time that the injectors are open.
Q: How does the controller know what to do?
A: By monitoring the primary Oxygen sensor(s) (pre catalytic convertor), engine coolant temperature,
throttle position, air mass volume, engine speed(rpm) and to a lesser extent changes in altitude, humidity,
ambient temperature, fuel quality, etc.