indicates that the contents of a particular field are not consistent across all environment implementations and must not be relied upon by UDI drivers.
When a value or behavior is unspecified, the specification defines no portability requirements for a facility on an implementation even when faced with a document that uses the facility. A document that requires specific behavior in such an instance, rather than tolerating any behavior when using that facility, is not a Strictly Conforming XHTML Document.
adj. Refers to behavior which is not fully specified by the Standard, for which each implementation must choose some behavior, though it need not be documented nor even consistent. Example: the order of evaluation of function arguments and other subexpressions. See question 11.33.