<xsl:key>

The <xsl:key> element declares a named key which can be used elsewhere in the stylesheet with the key( ) function.

xml

<xsl:key name=NAME match=EXPRESSION
  use=EXPRESSION />

name: Specifies a name for this key. Must be a QName.
match: Defines the nodes for which this key is applicable.
use: Specifies an XPath expression that will be used to determine the value of the key for each of the applicable nodes.

None.

Top-level, must be the child of <xsl:stylesheet> or <xsl:transform>.

XSLT, section 12.2.

Supported.