rootid-selection does not work for me

I'm using the rootid parameter with the modular XSL style sheets 
(1.60.1), saxon 6.4 and XEP 3.3 to produce a PDF of one specific chapter.

I've provided a valid ID as the value:

<chapter id="SimpleDataTypes">

and in the stylesheet, I have:

<xsl:param name="rootid" select="SimpleDataTypes"/>

but it always
formats the entire XML instance.
What's wrong here?

Joachim