[XeTeX] XeTeXpdffile media box etc

[Will Robertson]

Hi,

I'm writing up the syntax of \XeTeXpdffile and \XeTeXpicfile and I'd 
like someone who knows better than I do to explain the optional 
argument to \XeTeXpdffile to control the bounding box of the graphic:

     [ crop | media | bleed | trim | art ]

Any volunteers? (If these are standard "industry" terms, is there a 
good reference page to link to, perhaps?)

Also, is there a way to access this functionality through \includegraphics?

Thanks,
Will