A filter can only be defined if a filter condition is connected to
another element. Click on
Add in order to create a new filter.
A condition has to be connected to the output of an element, which
output shall be filtered before it is passed on to a further element. A
condition will first count as complete and correct once the connection
to both elements has been constructed. When creating the second
connection arrow, a dialog will open automatically in which you can
define whether the section between condition and following element must
always be run, or only if the condition
is fulfilled, or also if the condition is not
fulfilled.