The XSC contains 389 columns of information per source (most of which is photometry.) Because extended sources are much more complicated than a point source, it takes many more parameters to characterize them. Some parameters are required to describe the spatial extent of a source, and many more are required to characterize the photometry. In addition to a number of standard measures of photometry, the xsc also reports the magnitude as measured in a number of different fixed-size apertures. The columns are described in the tables below. We have separated the columns according to their basic function and utility.
Photometry algorithms and performance can be found here:
Large Aperture Photometry: Profile Fitting, Isophotal & Total Mags.
The XSC FAQ and Primer is here:
How to Use the 2MASS XSC -- FAQ Sheet.