The "High" output is for tweeters or small midranges. The "Low" output is for mid-bass drivers in your doors or rear deck. The "Sub" output is for your trunk subwoofer.
While Pioneer (now part of the Onkyo Home Entertainment group) generally does not keep manuals for units older than 10 years, it is still worth emailing their support team. Provide the model number and ask if a "Legacy PDF" exists.