skeleton 

The skeleton element is used to indicate where a skin controller is to start to search for 
the joint nodes it needs.  This element is meaningless for morph controllers.

Element Information

Model

Source 

<xs:element name="skeleton" type="xs:anyURI" minOccurs="0" maxOccurs="unbounded">
  <xs:annotation>
    <xs:documentation>The skeleton element is used to indicate where a skin controller is to start to search for the joint nodes it needs. This element is meaningless for morph controllers.</xs:documentation>
  </xs:annotation>
</xs:element>

Sample