The <xsl:when> element always appears within an <xsl:choose> element, acting like a case statement.
<xsl:when test=EXPRESSION>
TEMPLATE
</xsl:when>testSpecifies a boolean expression to be evaluated. If true, the contents of the element are processed; if false, they are ignored.
None.
Subinstruction, always appears within an <xsl:choose> element.
XSLT, section 9.2.
Supported.