Extration from Outlook Help:
'When you archive, your existing folder structure is maintained in your new archive file. If there is a parent folder above the folder you chose to archive, the parent folder is created in the archive file, but items within the parent folder are not archived. In this way, an identical folder structure exists between the archive file and your mailbox. Folders are left in place after being archived, even if they are empty'.
Hope that helps?