a0d929ca_WSRP_v2_0_Specification__Statement_text
[#] CS125 [Section] 7.14 [Statement text] 
While it is possible the set of properties can change with time (e.g. the Portlet dynamically creates or destroys properties), Producers and Portlets SHOULD make the returned modelDescription as complete as possible.
WSRP v2.0 Specification
Statement text