The superbias subtraction step removes the fixed detector bias from a science data set by subtracting a superbias reference image. Thisstep is only applied to the NIR instruments: NIRCam, NIRIRSS and NIRSpec. The 2-D superbias reference image is subtracted from every group in every integration of the input science ramp data. Any NaN’s that might be present in the superbias image are set to a value of zero before being subtracted from the science data, such that those pixels effectively receive no correction. The DQ array from the superbias reference file is combined with the science exposure PIXELDQ array using a bit-wise OR operation.
This link to the JWST pipeline provides detailed information on this step.