It provides hit markup info to the adobe acrobat browser plugin so that it knows what words to hilight. If you don't care about hit markup within the pdf documents you can omit it.
If you're curious you can submit the url after xml= with your browser to see what the data is.
Thanks, but when I try the URL after the xml=, I always get:
Warning: 'delimiters' not allowed in query
Warning: 'intersects' not allowed in query
My original search is "pressure". Looking over the code for this function (xml), I suspect that the $Body argument passed to pdfxml contains the offending chars, but I don't know how to fix this. If I supress the warnings, the query just dies quietly and returns empty XML markup.