See the SOI Tech Note SOI-TN-01-144 (PDF)
MDI Measurement Errors: The Magnetic Perspective by Y. Liu and A.A. Norton for details.Note that the magnetic field observed quantity is computed onboard as a difference between two "Dopplergrams" observed in left and right circularly polarized light. The onboard Dopplergrams have a single calibration curve applied for all pixels. Due to non uniform filter performance across the field, the Doppler zero and sensitivity vary from one part of the image to another. A velocity dependent correction is made on the ground for Dopplergram data. This can not be done for magnetograms since only the difference is brought to the ground. Thus we expect 10-30% sensitivity variations across the MDI magnetograms. An approcimate correction can be made based on average velocity across the disk or from comparisons to ground-based magnetogram observations. The "level 1.8" data has our best estimate of this correction at the time of data export. The "level 1.5" data has only the on-board computed magnetogram data with a scale factor to convert a difference in Dopplergrams to Gauss. "Level 1.8" data should be used whenever possible.
The current lev1.8 data is described at http://soi.stanford.edu/magnetic/Lev1.8/