Whenever multiple tags specifying a single attribute are presented in a <name>, <type>, or <case>/<undef>/<default> specification, those tags are joined into a fallback chain. If an entry in the chain fails, evaluation proceeds to the next entry. Only when the last entry of the chain fails is the failure propagated upward:
<name value='%a'/> <name value='%b'/> <name value='UNKNOWN'/>
If %a is defined, the name is its value. Otherwise, if %b is defined, the name is %b. Finally, if both are undefined, the name is UNKNOWN.