XSD Specification
 <xs:simpleType name="IfcActionSourceTypeEnum">
  <xs:restriction base="xs:string">
   <xs:enumeration value="dead_load_g"/>
   <xs:enumeration value="completion_g1"/>
   <xs:enumeration value="live_load_q"/>
   <xs:enumeration value="snow_s"/>
   <xs:enumeration value="wind_w"/>
   <xs:enumeration value="prestressing_p"/>
   <xs:enumeration value="settlement_u"/>
   <xs:enumeration value="temperature_t"/>
   <xs:enumeration value="earthquake_e"/>
   <xs:enumeration value="fire"/>
   <xs:enumeration value="impulse"/>
   <xs:enumeration value="impact"/>
   <xs:enumeration value="transport"/>
   <xs:enumeration value="erection"/>
   <xs:enumeration value="propping"/>
   <xs:enumeration value="system_imperfection"/>
   <xs:enumeration value="shrinkage"/>
   <xs:enumeration value="creep"/>
   <xs:enumeration value="lack_of_fit"/>
   <xs:enumeration value="buoyancy"/>
   <xs:enumeration value="ice"/>
   <xs:enumeration value="current"/>
   <xs:enumeration value="wave"/>
   <xs:enumeration value="rain"/>
   <xs:enumeration value="brakes"/>
   <xs:enumeration value="userdefined"/>
   <xs:enumeration value="notdefined"/>
  </xs:restriction>
 </xs:simpleType>