XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (518 page)

IdKeyPattern

This construct allows a pattern to be matched only if the node being tested (or one of its ancestors) has a specified ID attribute or key value.

This construct is a subset of the
FunctionCall
construct in an Expression, described in Chapter 7. The only function calls that can be used in a pattern (except within predicates) are the
id()
and
key()
functions, and these can be used only with arguments that are literals or variable references.

The
id()
function is an XPath function, while the
key()
function is exclusive to XSLT. Both are described in Chapter 13.

Syntax

Expression	Syntax
IdKeyPattern	id ( Value ) | key ( StringLiteral , Value )
Value	Literal|VariableReference