It is part of the data stored by the car's computer relating to the O2 sensor. Technically it is byte "B" of PID 14 of the Mode 1 OBDII commands:

"PID 14 Oxy. sensor voltage bank1 sensor1
This PID returns the Oxygen sensor voltage for the sensor located in Bank 1, Sensor 1. There are 7 other PIDs which are similar, but for other locations. In each case, Two bytes are returned. Data A represents the sensor voltage (defined below), and Data B is the short term fuel trim associated with the sensor, or Hex FF in the case where the sensor is not used in the calculation.
Oxygen Sensor Voltage = .005 * A
Short Term Fuel Trim% = .7812 * ( B-128 )"

Other sites referred to this as the "calculated trim". In any case it is not same as the the Short Term fuel trim associated with the bank or the Long Term fuel trim. Those are PID 6 and 7:

"PID 06 Short term fuel % trim Bank 1
The fuel trim is a value from -100% (lean) to +99.22% (rich)
Fuel Trim% = .7812 * ( A-128 )

PID 07 Long term fuel % trim Bank 1"

This is from http://obddiagnostics.com/obdinfo/pids1-2.html

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra