XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (405 page)

In effect, the predicate operator
[
]
has higher precedence (it binds more tightly) than the path operator
/
.

Another distinction between the two cases is that in the case of a
FilterExpr
, the items are always considered in their original order when evaluating the predicate. In the case of an
AxisStep
, the nodes are considered in the order of the relevant axis. This is explained in more detail below.

A predicate may be either a boolean expression or a numeric expression. These are not distinguishable syntactically; for example, the predicate
[$p]
could be either. The distinction is only made at runtime. (That's the official rule, anyway. If an optimizer can work out in advance whether the value is numeric or boolean, then it will. It's a good idea to declare the types of your variables and parameters, which will make the optimizer's job easier.)

The following table shows some examples of boolean predicates:

Expression	Description
section[@title = ‘Introduction’]	Here the predicate is a conventional boolean expression. This example selects every child element that has a title attribute with the value Introduction .
section[title]	The predicate is true if the relevant section has at least one child