Hi Paul,
You can find more information on this here : http://pubs.vmware.com/vcloud-api-1-5/wwhelp/wwhimpl/js/html/wwhelp.htm#href=api_prog/GUID-43DFF30E-391F-42DC-87B3-5923ABCEB366.html
This is the 1.5 doc but there must be the 5.1 equivalent.
Christophe.