In a regular table layout, the filtering of the data is done through the selected element types. You select a set of metaclasses on the Element Types tab when you set up your table layout. When you view your table, the table shows you the elements of those metaclasses that are in the defined scope for that table view.
For an extended table layout for a relation table, the filtering functionality has been extended so that in addition to the element type filtering, you can include filtering the elements by the metaclass of their "form" and "to" elements. For information about creating extended table layouts, see Creating extended table layouts.