It reads off the PCM and can display 2 things at once. It can display almost anything the PCM reads. Heres a sample list of stuff it will display:
RPM
MPH
Intake Air Temperature (post Intercooler)
Ambient Air Temperature (pre Intercooler)
Engine Coolant Temperature
Exhaust Gas Temperature (PCM Calculated)
Transmission Temp. (auto trans)
Manifold Pressure (vacuum/boost) displayed in PSI
Fuel Temperature
Battery Voltage
Battery Temp
Injector Pulse Width in msec.
Injector Duty Cycle (%)
Air Flow Rate into engine
Long Term Fuel Trim
Short Term Fuel Trim
O2 Sensor Voltage
Barometric Pressure
Ignition Advance
Base Spark
PCM Spark Adjust
Fuel Status (open/closed loop)
Wastegate Solenoid Duty Cycle (%)
Knock Retard
Knock Sensor Raw Voltage
Throttle Position Percentage
Throttle Position Sensor Raw Volts
Calculated Net Horsepower
Engine Load
Transmission Slip
Current Transmission Gear
Torque Delivered to Transmission
Torque Converter lockup status
Brake pressure
Brake Booster Travel
Steering Angle
Non-driven Wheel Speed
Lateral G's
Yaw
Fuel economy (instantaneous and average)
P-ratio - pressure ratio MAP/Barometer
2 user configurable analog inputs**