XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (303 page)

For a more complete explanation of this distinction, see the sections on
Axis Steps
in Chapter 9 and
Filter Expressions
in Chapter 10.

Another rather specialized use for parentheses is to remove syntactic ambiguities when using the
/
symbol as an expression referring to the document node at the root of the current tree. When
/
is followed by a name, or by the symbol
*
, then it is assumed to be the start of a rooted path expression. This means that if you want to follow
/
with a named operator, you need to enclose it in parentheses, for example:

or

Another way of disambiguating
/
in such expressions is to write it as
/.
.

Changes in XPath 2.0

The syntax
()
to represent an empty sequence, and the use of the
,
operator to perform sequence concatenation, are new in XPath 2.0.