This selects which mailbox format will be the default for local folders: mbox: KMail's mail folders are represented by a single file each. Individual messages are separated from each other by a line starting with "From". This saves space on disk, but may be less robust, e. g. when moving messages between folders. maildir: KMail's mail folders are represented by real folders on disk. Individual messages are separate files. This may waste a bit of space on disk, but should be more robust, e. g. when moving messages between folders.
continuation of "By default, & message folders on disk are"