A match structure upon which the relative photometry algorithm (Section 8.3.3) has been applied still contains the exact same structure as the original match structure (although some of the data values in that structure have been changed, of course). What is new is a third FITS file extension has been added, to preserve useful information about the correction process. This third extension is a single structure with the following fields:
NPIX Number of sub-fields used
NXBIN
NYBIN
NTHR
NOISE
OFFSET Array of mean offsets per subfield
ERROR
NTMP
SDV