[Xapian-discuss] Can someone please explain this paragraph?

[Jim]

At http://xapian.org/docs/intro_ir.html, near the bottom of the document 
there is a paragraph with the heading of "The progress of a query".  In 
that paragraph there is a reference to something called an RSet.  I 
can't find a definition of that term in this document. 

Perhaps someone could explain what it is and/or expand on it in the 
.html file too?

Thanks,
Jim.