If auditing is enabled for the event by setting an event level greater than 0, then this field can optionally control the criteria under which the event should fire.
The meaning of the criteria is dependant on the type of event. An example for database connections could be a comma delimited list of databases that you are interested in auditing the connections for. If a database is connected that does not meet the criteria, then no audit data will be generated.
As stated, this is optional additional criteria and only relevant if the event auditing is enabled.