#include <libmail/mail.H> mail::folder *folder;
Some folders contain messages. Other folders contain other folder (subfolders). Some IMAP servers are capable of creating dual-purpose folders that contain both messages and other subfolders. The hasSubFolders indicates whether folder is capable of containing other folders (subfolders).
A true result indicates that folder is capable of storing other folders. It may not necessarily mean that folder actually contains any folders, it might very well be an empty folder.
Applications should interpret a true result to mean that the mail::folder::createSubFolder(3x) function may be used to create subfolders.