Add the following section to Chapter 3, "Notations and Terminology". Note this issue is specific to WS-Eventing, but the following proposed text may be suitable for the other WS-RA specs as well (once the proper issues have been raised, etc.)

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3.x Considerations on the Use of Extensibility Points

The elements defined in this specification MAY be extended at the points indicated by their outlines and schema. Implementations MAY add child elements and/or attributes at the indicated extension points but MUST NOT contradict the semantics of the parent and/or owner, respectively. If a receiver does not recognize an extension, the receiver SHOULD ignore that extension. Senders MAY indicate the presence of an extension that MUST NOT be ignored through the use of a corresponding SOAP Header with a soap:mustUnderstand attribute with the value “1”.