Arduino 9 Axes Motion Shield - Calibration Status Data

Does anyone know how to interpret the 'Calibration Status' data ( for the Accelerometer, Gyroscope and Magnetometer) which is written to the Serial Port (along with the Euler angles), when running the 9 Axis Motion/ Euler.ino?

Snippet from the example sketch;

Serial.print(mySensor.readAccelCalibStatus()); //Accelerometer Calibration Status (0 - 3)

There's no explanation in the sketch or in the Product info that I can find, so any information would be appreciated.