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The optional character of a specified feature, when concerning an artifact that may be produced and consumed, is a common source of confusion and interoperability failure. An artifact MAY implement a feature. This clearly means that a compliant device producing such an artifact MAY omit this feature. However, any compliant device consuming such an artifact MUST be able to process this feature, should it be present (unless specified otherwise). In this way useful or important information included in the optional feature, is not omitted for the receiving application.
Interoperability & servicedesk
The servicedesk of the implementer should be able to help the end-user beyond the boundaries of the own implementation. If the root cause of the incident is found or can only be serviced in a third party's implementation, then one of the servicedesk of that third party or the writer of the specification should be informed. Otherwise the end-user reporting the interoperability incident will not be serviced and a possibly underlying problem will not be solved.