If the doubleclick option is switched on (refer to ) you can do different operations with a doubleclick.
If you doubleclick on a subdirectroy it will be loaded in the particular directorywindow. A doubleclick on a entry in the devicewindow has a simular result the contents of the selected volume, physical or logical device is loaded in the activated directorywindow.
If you doubleclick on a file Filer selects automatically a operation to do:
If Filer doesn't know what to do with a file it'll show an error requester.