If you have a rule you want to apply to another folder, you can copy it (and then, if necessary, make any necessary modifications to the copy). You can also move a rule to a new folder (removing it from the first one in the process).
To copy a rule to another folder:
Drag the rule from the “Rules” list to the desired folder in the “Folders” list.
Drop the rule on the folder to add the rule to the end of that folder’s rule list. Alternatively, you can drag the rule to a specific position in the rule list for the destination folder. A horizontal line appears where it will be placed.
To move a rule to another folder:
Follow the steps above, but hold down the ⌘ key during the drag and drop. The rule disappears from the original folder.
Note: Rules that are copied or moved between folders are initially disabled in the new location to prevent cases in which incorrect matches would occur if the rule were not edited first. After you edit the rule (if necessary) in its new location, select its checkbox to enable it.