MUST vs. MAY in CMS specification

    I have to admit to some confusion over the claimed need to
back the requirements for digested-data, encrypted-data, and
authenticated-data from MUST to MAY.  My reading of RFC 2630
suggests that they already are MAY, and neither are they used in
the message spec (RFC 2633).  Clause 2 of CMS is pretty clear in
stating the conformance requirement for CMS.  To wit:

     An implementation that conforms to this specification
     must implement the protection content, ContentInfo, and
     must implement the data, signed-data, and
     enveloped-data content types.  The other content types
     may be implemented if desired.

Can somebody explain where the supposed requirement to support
these wrappers stems?

Chris

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