Undocumented use of
You are using an extension element in a context for which basic constraints (e.g. cardinality, uniqueness) and semantics for this element have not been defined.
Check to see if this element is misplaced. If you misplace an element, this information may not be recognized by feed consumers.
If the element placement is intentional, be aware that the feed validator is unable to verify the correct usage of this element in this context. Also be aware that this usage is not likely to be widely supported.
If you know of a extension element usage that you feel that the feed validator should support, please let us know.
Not clear? Disagree?
You might be able to find help in one of these fine resources.