Go to standardXSLT function: node-set key (string, object)
Standard excerpt:
The key function does for keys what the id function does for IDs. The first argument specifies the name of the key. The value
of the argument must be a QName. When the second argument to the key function is of type node-set, then the result is the
union of the result of applying the key function to the string value of each of the nodes in the argument node-set. When the
second argument to key is of any other type, the argument is converted to a string as if by a call to the string function;
it returns a node-set containing the nodes in the same document as the context node that have a value for the named key equal
to this string.
Examples (xslt:xml): Overview
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54:12
55:12Interactive xlab: [xsl:key] [xsl:value-of] [xsl:template]