It is stated in section 5.7.1 p:variable that:
"The variable's value is specified with a select attribute. The select
attribute must be specified. The content of the select attribute is an
XPath expression which will be evaluated to provide the value of the
The section then goes with statements such as "If a select expression is
given...". Since the select attribute is mandatory, the select
expression must necessarily have been given.
Agreed. The text that suggests that the expression may not be given is an editorial error.