When saving an Acces document, user-created content (such as user-created images or user-edited passages) is referenced within the document. The files, however, are stored in your user folder. The document will open fine on the same computer it was created on, but may have "Missing File" errors when opened on another computer.

Exporting a Document Package solves this problem by packaging user-created figures, passages, etc. along with an Acces document. Other Acces users who open a document package will see the document as it was originally created: user-created content and all.