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The "m" indicates the images were bias corrected, which (if the option is selected) is done in the SPM "unified" procedure that runs with the segmentation. SPM terms this creating "modulated" images, hence the letter m. Bias correction adjusts for major intensity differences across the fMRI images; I found in one dataset that one side was much darker than the other, which is why I added the option to correct. This darker/brighter is often seen in raw T1 images as brighter or darker spots.
