Q’s about http-request attributes

So, I keep referring back to the XProc specification for stuff like this, but I can’t seem to find the answer anywhere.  A lot of attributes appear to be just plain unexplained.  

So, here goes my list of questions for p:http-request:


@doctype-public and @doctype-system 
	What is their purpose?

@include-content-type and @media-type : 
	Is there a difference between “content type” and “media type”?  Or are these both MIME types?  

@standalone : 
	I have searched and searched for the purpose of this attribute with no luck.  The specification is nice enough to tell me that it takes true, false, and omit, but not what those values actually do.

@version : 
	Does this refer to HTTP version 1.0 / 1.1?  Or to some other version?



Thanks in advance,

—Tony