If user declares top-level parameter serna.stylesheet.dir, then Serna XSLT processor will set this parameter to the directory name of the top-level stylesheet. This is often necessary when some fixed set of images (e.g. icons) are used together with the stylesheet. See the example below:
<xsl:param name="serna.stylesheet.dir"/>
....
<xsl:template name="show.icon">
<xsl:param name="icon.name"/>
<fo:external-graphic
src="{concat('url(', $serna.stylesheet.dir,
'/icons/', $icon.name, '.png)')}"/>
</xsl:template>
Defining the value of serna.stylesheet.dir is possible. This value will be always redefined by Serna XSLT processor, but can be used e.g. as default for other XSLT processors.