What are the secret options of the PDF convert tool from OS X?

Question

I recently discovered the /System/Library/Printers/Libraries/convert tool that is able to convert HTML, DOC, ... to PDF.

It works great but it has some optional arguments that are not documented: -a 'name=value ...'. I am really interested about them because these are supposed to control more of the 'printing' process.

/System/Library/Printers/Libraries/convert
Usage: convert [ options ]

Options:

  -e                   Use every filter from the PPD file
  -f filename          Set file to be converted (otherwise stdin)
  -o filename          Set file to be generated (otherwise stdout)
  -i mime/type         Set input MIME type (otherwise auto-typed)
  -j mime/type         Set output MIME type (otherwise application/pdf)
  -P filename.ppd      Set PPD file
  -a 'name=value ...'  Set option(s)
  -U username          Set username for job
  -J title             Set title
  -c copies            Set number of copies
  -u                   Remove the PPD file when finished
  -D                   Remove the input file when finished