The third parameter is useful when you search for a text against a property of the list items.
For example, if each item in the list (say User object) containing a property named 'role', then you can search the percentage of users who belongs to manager role.
Conversly, if the list items are simple types such a List<String>, you cannot search a value against any property name. This will cause an error. However you can directly search for it's value and find the percentage.
Adobe Documentation explaining this function usage:
http://help.adobe.com/en_US/livecycle/9.0/workbenchHelp/help.html?topic=000097&topic=002025
Nith