Each location step can contain zero or more predicates that further restrict which nodes an XPointer points to. In most non-trivial cases a predicate is necessary to pick the one node from a node set that you want.
Each predicate contains a boolean
expression in square brackets ([]) that further
winnows the node set.
This allows an XPointer to select nodes according to many different criteria. For example, you can select:
All elements that have a specified attribute
All elements that have a specified attribute with a specified value
The first element that contains a specified child element
An element whose text content includes a specified string
All elements that are not the first or last children of their parents
All elements whose value is a number
All elements whose value is a number greater than 100