An "Edit Mask" or "Edit Filter" window appears when you respectively need to specify a mask or filter. The window contains two lists. The list at the left shows the rules that are to be used in the mask or filter. The list at the right shows all rules that are still available. You can select rules that should be used by moving them from the right to the left, using the < button.
When there is more than one rule in the mask (or filter), a file only needs to match one for the mask (or filter) to apply. In other words, the rules are combined using "OR" as opposed to "AND".