If empty, the job will be triggered for any event occuring on the selected node. The filter expression allows the job to be triggered only if the event matches a certain criterium. The filter is a boolean expression in XPath notation, which is applied on the XMPP event. If the boolean expression evaluates to true, the job is triggered. See XEP-0060 for more information about XMPP Pub/Sub.
TODO: add some examples