Folders can themselves be grouped together inside objects called containers. A container is an object that holds folders, much in the same way that a folder holds messages. As for folders, the concept of a container is a generalization of the different kinds of mechanisms used by the underlying mail technology. So, for example, the container of a file folder is the directory holding that file.
IMAP containers are a little different: each IMAP mailbox is capable of holding other mailboxes. What this means is that, from IMAIL's point of view, an IMAP mailbox is both a folder object and a container object at the same time. However, when IMAIL views an IMAP mailbox as a container, it is treated differently than when it is viewed as a folder, and consequently different notation is used in each case.