In the Open dialog box, you can select documents, presentations, or both. Multiple files are opened in the order that the Selection frame lists them. The order of documents inside the presentation is observed in the context of the other files being opened.
If multiple presentations contain multiple instances of a document, only the first instance is opened. It may seem that the document did not open or that documents opened in the wrong order. Only one instance of a document can be opened (and modified) at a time.