External Folders / Dropbox
Ulysses is a library-based app; all of the contents in the iCloud and On My Mac/iPad/iPhone sections live in this library. However, you can also embed external folders in Ulysses that are stored on your local hard drive or on a cloud service, and make their contents behave like native groups and sheets. You can access these folders via Finder, and depending on the chosen format, even open the contained files with third-party apps.