Go to standardXPath function: node-set id (object)
Standard excerpt:
The id function selects elements by their unique ID. When the argument to id is of type node-set, then the result is the union
of the result of applying id to the string-value of each of the nodes in the argument node-set. When the argument to id is
of any other type, the argument is converted to a string as if by a call to the string function; the string is split into
a whitespace-separated list of tokens (whitespace is any sequence of characters matching the production S); the result is
a node-set containing the elements in the same document as the context node that have a unique ID equal to any of the tokens
in the list.
Examples (xslt:xml): Overview
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76:16Interactive xlab: [xsl:value-of] [xsl:template]