Embedding files versus creating aliases

By default, when you attach a file to an outline it gets embedded into the outline. This means that a copy of the attached file is actually put inside the outline file itself. If you subsequently move the file around, or send it to someone else, the attached file will move with it.
If you attach a folder, or hold the control key while dropping a file, an alias (a "link") to the file or folder will be created instead. The alias behaves just like an alias in Finder, as a pointer to the actual file. This means you can change the file or move it around, and OmniOutliner will try to follow the changes. Of course, if the file is deleted, gets moved to a network drive that is then disconnected, or otherwise goes where OmniOutliner can't find it, the file reference in the outline will become useless. So, if you plan to send an outline to someone else, or otherwise move an outline to another computer, you should embed your file attachments instead of making aliases to them.