It’s not clear what the relationship between Required and MustUnderstand attributes in a Choice element would be. Any miss-match between the values of the two is likely to trigger an error. Given the provision of a FallBack element, it is not clear there is any use case where there’s a value in the use of MustUnderstand in a Choice.

Revise the text to address this comment

9.2.2 [16]

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Proposed Disposition of DIS 29500 Comment GB-0615 (Modified: 2008-01-04) Agreed; the reasoning for this was not made clear. The MustUnderstand attribute and the Ignorable attribute are designed as complementary mechanisms to control markup compatibility processing of descendant elements. The Ignorable attribute does have a great deal of utility on the Choice element, since the Choice element may have many sibling children without a single common parent whereupon to introduce the Ignorable namespace specification. It would be unusual and a burden on markup consumers to special-case their handling of the Ignorable/MustUnderstand attribute pair for this one element to prohibit the MustUnderstand attribute on the Choice element. The following note has been provided to explain this reasoning. Part 5, §9.2.2, page 21, lines 16­17: Choice elements can include the attributes Ignorable, MustUnderstand, ProcessContent, PreserveElements, and PreserveAttributes described in this specification. [Note: Although the MustUnderstand attribute might seldom appear on a Choice element, it is permitted there to avoid requiring markup consumers to special-case their handling of the MustUnderstand and Ignorable attribute pair. end note]