For those developers that use the freely distributable math library, or fdlibm, a conflict exists between the definition of UNKNOWN. Specifically, if you include fdlibm headers prior to xerces-c headers, xerces-c user applications will not compile.
Choosing common names such as UNKNOWN is not helpful to those of us that link in many third-party libraries, since the likelihood that a conflict will exist with such a name is greatly increased. Please prefix the names with something that distinguishes the enum value as being from Xerces-C. In the very least, set it as a standard practice in future enum types to use more conspicuous names. It would be nice if you changed these before these names get entrenched in C++ user code.
I do have a work-around, but it's rather ugly. I just raise this to your attention, mostly to suggest the adoption of such a coding practice.