When I use these different kinds of calls I get the same (good)results. Does anyone know if there is a difference and why and when to use them:
   <xsl:value-of select="document('reference.xml' )/key('ref' , current()/value )" />
      <xsl:value-of select="doc('reference.xml' )/key('ref' , current()/value )" />
      <xsl:value-of select="key('ref' , current()/value,document('reference.xml' ))" />
      <xsl:value-of select="key('ref' , current()/value,doc('reference.xml' ))" />

I should mention for other readers that these are all XSLT 2.0 constructs.

The differences between doc() and document() are mainly (a) that document() can take a sequence of URIs and return a sequence of documents, whereas doc() only handles one, and (b) that document(), if given a node rather than a string as its argument, resolves relative URIs relative to the base URI of that node, rather than the base URI of the stylesheet.

The differences between $node/key(A, B) and key(A, B, $node) are mainly that (a) the context node for evaluation of b is different for the two cases, and (b) the second form searches the subtree rooted at $node, not the whole document.

In your cases these distinctions make no difference to the result.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference

Thanx for this information this is a great help. for understanding the differences

I still have some differences when I use absolute paths with http: protocol.
Before I want to get a value from an external xml document with this key construction, I want to test if it is a really available document.
But I never seem to get a true value when I use the new (version 2.0) doc-available function for absolute paths with the http: protocol (When I am testing a local file with the file protocoI )

<xsl:if test="doc-available('http://www.mydomain.nl/xsd/mylist.xml')" > is always false
but the relative paths like
<xsl:value-of select="doc-available('../xsd/mylist.xml')" /> is true.
And the absolute path (on my mac)
<xsl:value-of select="doc-available('file:/Volumes/www/xsd/mylist.xml')" /> is true.
and also when I use the resolve-uri() function for creating absolute paths for local files

how come? does doc-available not recognice http:paths when testing locally?

(a) Which XSLT 2.0 processor are you using?

(b) Is your site behind a firewall?

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference

The difference is in the environment.
When I do the same test on a windows computer with the same processor saxon8 on windows doc-available() finds the http: referenced documents and on my mac I get the doc-available false boolean as result. I've shutdown the firewall on my mac for it. anyone else have that problem?