Selection criteria type, which is a Unified CM Selection criteria, follows the SOAP header. Selection criteria should not include "unknown." If you specify "unknown," it is treated as "any." The "Unknown" state can be present in a response.
Note, however, that if the device has not been seen (registered or whatever) since the last reboot, then selectCmDevice won't find it even if you specify "Any" as the search criteria. If the device has been "seen" (for example, registered), then it will show up, even if it is not currently registered.