The catalogue include essentially all galaxies north of declination -02°30' and to a limiting diameter of 1.0 arcminute or to a limiting apparent magnitude of 14.5. The primary source of data are the blue prints of the Palomar Observatory Sky Survey (POSS). Galaxies smaller than 1.0 arcminute in diameter but brighter than 14.5 magnitude may be included from the Catalogue of Galaxies and of Clusters of Galaxies (CGCG).
The catalogue contains descriptions of the galaxies and their surrounding areas, plus conventional system classifications and position angles for flattened galaxies. Galaxy diameters are included and the classifications and descriptions are given in such a way as to provide as accurate an account as possible of the appearance of the galaxies on the prints. The accuracy of coordinates is only what is necessary for identifications purposes.
Reference
http://nedwww.ipac.caltech.edu/level5/March02/Nilson/Nilson_contents.html