Syntax:
This creates a reduced catalogue, based on a selection expression. The original catalogue is moved to a new name. If the original catalogue was xsel_s1.cat, then it will be moved to xsel_s1_v.cat. The new obscat will have the name xsel_s1.cat, and will be automatically loaded for you. So choose and show obscat will now act on this catalogue. N.B. this command does not read in any data, it only produces a reduced catalogue. You will now have to choose data from it.
xsel > select obscat <cat_sel>
Example:
xsel > select obscat ''datamode=='BRIGHT'''
This makes a reduced obscat containing only files for which the datamode is BRIGHT.