Where is wintitle used?

Question

I'm a DITA-OT newbie. I've recently installed version 3.1.2 and customized the wintitle element in ui-domain-attr.xsl of my custom plugin folder as follows:

<xsl:attribute-set name="wintitle">
    <xsl:attribute name="font-weight">bold</xsl:attribute>
    <xsl:attribute name="font-style">italic</xsl:attribute>
</xsl:attribute-set>

When I render a pdf based on the document sequence.ditamap(which is part of the dita-ot folder), I see no change. What am I doing wrong. PS: I have also updated custom.xsl(attr)