I am actually referring to an iPAD or iPhone connected to the RC unit.  Sometimes, but not always, when I start everything up and go through the standard GPS calibration for the drone I also get a "calibration request" for the iPAD or iPhone with a circular dial showing up inviting my to rotate that device through 360 degrees.  Can anyone explain what this is calibrating and why as it seems to refer specifically to that device rather than the Phantom.  So far I have, on most occasions, ignored and cancelled it with no detriment to flying the Phantom.  I nevder see this calibration request other than when I am using DJI GO.

Hello Brian, the rotating of IOS devices is a way to calibrate the IOS device's compass. I'd suggest you to finish this calibration.