With respect to the GetMetadata request - given a particular set of metadata (at a service) and a particular set of @Dialect elements and attributes (in a request), it is possible for a GetMetadata request to effectively narrow down the set of returned metadata sections to the empty set - i.e. there is no metadata matching the requester's selection criteria. The spec is silent about what happens in this case.

Proposal - add the following sentence to the end of the description of [Body]/mex:GetMetadataResponse/mex:Metadata:

"Note, in cases where the requester, through the use of @Content and/or mex:Dialect sub-elements in the GetMetadata request, has excluded all the metadata maintained by the service, this element will be empty."