The following are the properties relating to folders:
PR_ASSOC_CONTENT_COUNT
PR_COMMON_VIEWS_ENTRYID
PR_CONTENT_COUNT
PR_CONTENT_UNREAD
PR_DEFAULT_VIEW_ENTRYID
PR_FINDER_ENTRYID
PR_FOLDER_TYPE
PR_PARENT_ENTRYID
PR_STATUS
PR_SUBFOLDERS
PR_VALID_FOLDER_MASK
PR_VIEWS_ENTRYID
For more information on folders, see Folders.