<xsl:when>

The <xsl:when> element always appears within an <xsl:choose> element, acting like a case statement.

xml

<xsl:when test=EXPRESSION>
  TEMPLATE
</xsl:when>

test: Specifies 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.