Answer
When viewing a user's archive store using the Folder View within the Archive tab, the meanings of the various folder icons are as follows:

Inbox: The standard Inbox folder icon as seen from the Folder View.

Sent Items: The standard Sent folder icon as seen from the Folder View.

Deleted Items: The standard Deleted Items / Trash Folder icon as seen from the Folder View. Please note that this folder will not appear by default, the option to synchronize the Deleted Items folder from must manually enabled,
as seen here.

Sub-Folder: Standard Sub-Folder icon as it appears within the Folder View of the Archive tab. Please not that the folder structure will not appear unless enabled for GFI Archiver,
as seen here.

Read-Only Folder: Folders will appear as Read-Only if contained within an archive store set to Read-Only access.