When producing a set of inter-related documents, you'll often want to refer to labels in another document of the set; but LaTeX, of its own accord, doesn't permit this.
So the package xr was written: if you say
\usepackage{xr}
\externaldocument{volume1}
will load all the references from volume1 into your present
document.
But what if the documents both have a section labelled
"introduction" (likely enough, after all)? The
package provides a means to transform all the imported labels, so you
don't have to change label names in either document. For example:
\usepackage{xr}
\externaldocument[V1-]{volume1}
loads the references from volume1, but prefixes every one with
the string V1-. So you would refer to the introduction to volume 1
as:
\usepackage{xr}
\externaldocument[V1-]{volume1}
...
... the introduction to volume1 (\ref{V1-introduction})...
To have the facilities of xr working with
hyperref, you need xr-hyper. For simple
hyper-cross-referencing (i.e., to a local PDF file you've just
compiled), write:
\usepackage{xr-hyper}
\usepackage{hyperref}
\externaldocument[V1-]{volume1}
...
... the \nameref{V1-introduction})...
and the name reference will appear as an active link to the
"introduction" chapter of volume1.pdf.
To link to a PDF document on the Web, for which you happen to have the .aux file, write:
\usepackage{xr-hyper}
\usepackage{hyperref}
\externaldocument[V1-]{volume1}[http://mybook.com/volume1.pdf]
...
... the \nameref{V1-introduction})...
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=extref