it's especially useful for the metadata-provider add-on. so you can prioritize the found constraints. anyway, it isn't bound to the other add-on - so you can use it independently.
(or impl. your own add-on - it's quite simple)
simple example:
you can find the add-on at:
http://code.google.com/p/os890/source/browse/#svn/trunk/java/web/jsf/extval/metadata_priority
