XPath predicate expressions are ultimately converted to a boolean after all calculations are finished. Non-boolean results are converted as follows:
A number is true if it's equal to the position of the context node, false otherwise.
An empty node set is false; all other node sets are true.
An empty result fragment is false; all other result fragments are true.
A zero length string is false; all other strings are true (including the string "false")
The predicate expression is evaluated for each node in the context node list. Each node for which the expression ultimately evaluates to false is removed from the list. Thus only those nodes that satisfy the predicate remain. I will not repeat here the discussion of the operators and functions available to use expressions. However, I will show you a few examples of predicates using the expression syntax as it's likely to be used in XPointers.