Normally you will choose to read the calibration. Calibration makes the data collection system accurate. The rail measurement system is calibrated in the field, and its calibration files remain there until it is re-calibrated. Because the calibration is needed by the Office System Read Run process, the calibration in effect at the time of data collection is included in the 'raw data'.
However, there may be a situation in which you do not want to use the calibration that was in effect when the data was collected. Sometimes, although the Field System is known to be out of calibration, it is not convenient for the crew to recalibrate before a data collection run. They recalibrate afterwards instead. In this case, you will want to apply the later calibration to the earlier data collection run. You do this by choosing to read Calibration for a later run (after the system was recalibrated), then reading the earlier run with the Calibration checkbox de-selected.