I found it in some legacy code I was given to update (just a large CGI). At first I thought it was just for error checking because there was a corresponding hash of error codes mapped to each of the form fields -- that was until I saw this:
<form method="post" ... action="<?PP:CGI-URL?>">
So my question is, what is that <?pp:...?> all about? Has anybody else seen it? Is it commonly used, and if so, where?