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The element <forLetClauseItemExtensions> is defined in the XQueryX schema as follows: <!-- Represents all variable bindings in a for or let clause except typed and positional variable bindings --> <xsd:element name="forLetClauseItemExtensions" abstract="true"/> Beyond the comment there is no documentation for this element at all. It is used in the schema in both the forClauseItem type and letClauseItem element, although it is an abstract element with no substitution groups defined. None of the XQTS cases use this element either.
It turns out that the schema in the specification (at http://www.w3.org/TR/xqueryx/) is wrong - this has been fixed in the schema found at http://www.w3.org/2005/XQueryX/xqueryx.xsd
Actually, the schema in the XQueryX Recommendation (http://www.w3.org/TR/xqueryx/) is the correct schema; for reasons we have not been able to explain to ourselves, the schema document at http://www.w3.org/2005/XQueryX/xqueryx.xsd is an obsolete schema document left over from an earlier Working Draft. We will correct that error shortly. The reason for the "undocumented" abstract type has to do with providing a mechanism for defining extensions to XQueryX in other documents under development (e.g., Full-Text and the Update Facility). Therefore, it is not a "bug", but a deliberate feature. As such, I am marking this bug resolved (INVALID) and ask that you mark it CLOSED at your earliest opportunity.