Folders_and_Folder_Collections

   The Pine view is that messages are stored in folders, and
   folders are stored in folder collections. Folder collections
   can be physically located on the local system, or on any remote
   system with an IMAP server. Regardless of what system a folder
   collection is physically located on, a Pine user sees it as just
   another folder collection: a Pine user can read messages in any
   of their folder collections and can save (move) messages between
   different folder collections.
 

Additional information available:

Folders         Collections