The description of [Body]/wse:Subscribe/wse:Filter/@Dialect contains the following paragraph:

"While an XPath predicate expression provides great flexibility and power, alternate filter dialects MAY be defined. For instance, a simpler, less powerful dialect might be defined for resource-constrained implementations, or a new dialect might be defined to support filtering based on data not included in the notification message itself. If desired, a filter dialect could allow the definition of a composite filter that contained multiple filters from other dialects. New dialect definitions MUST include sufficient information for proper application. For example, it would need to include the context (which data) over which the filter operates."

Most of this material would be more appropriately place in the primer.

Proposal:

"The XPath 1.0 dialect (described below) MUST be supported. Alternate filter dialects can be defined. Such dialect definitions MUST include sufficient information for proper application."