XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (435 page)

If the value of the predicate contains nodes, there is no automatic atomization of the nodes (that is, the values of the nodes are not extracted). In fact, if the value of the predicate contains one or more nodes, then its effective boolean value is always true. This means, for example, that
person[@isMarried]
selects any

element that has an
isMarried
attribute, irrespective of the value of that attribute. If you want to test the value of the attribute, you can atomize it explicity using the
data()
function, or you can use a comparison such as
person[@isMarried = true()]
. (But beware, if backward-compatibility mode is switched on, then
person[@isMarried = true()]
means the same as
person[@isMarried]
).

A numeric predicate whose value is
N
is equivalent to the boolean predicate
[position()
eq N]
. So, for example, the numeric predicate
[1]
means
[position() eq 1]
, and the numeric predicate
[last()]
means
[position() eq last()]
.