An advance sheet is a soft bound volume which is published prior to the hard bound volume.  For instance, the last published volume of the Pacific Reporter (above) is volume 945.  The next hard bound set of volumes may be two to six months away from being published.  In the meantime, the publisher sends the soft bound advance sheets, which contain the exact same material that will eventually appear in the hard bound volume when it is published.  When the hard bound volume is received, the advance sheets are thrown away.  (Also see Shepard's Advance Sheets.)