This is a required setting; if left blank, no data will be returned. It establishes a connection to your filtered data source connection. This property also supports the Common Use Variables stated at the beginning of this help topic.
Note: The Workplace Lookup will pass a query string parameter "filter=" with the value of the search filter on the right side of the equation. If using the External Data Provider, you will need to make sure you have provided this parameter as part of the value nodes.