Re: use-when in the xsl:inlcude

<xsl:include href="..." use-when="document('configuration:data')/data/xs:boolean(@xxx)"/>

Except that it won't work. Your use-when= above relies on *some* document context in order to determine the argument values for the document() function...

As you might understand, I have some configuration defining xslt to run.
E.g. Andrew has proposed to define all required compositions.
I was thinking of conditional xsl:include.

This works (if I'm not wrong), as doc() function is available
in the context where values of @use-when are evaluated.
See http://www.w3.org/TR/xslt20/#conditional-inclusion.