When you choose the Select from Page Attachments option while adding a 
Simple Diagrams macro, you'll be able to select any existing diagram attached to the page with the .drawio extension, insert it into the page, and open it for editing.
When you click the Select from Page Attachments button, a list of page attachments with the .drawio extension will be displayed:
Select a diagram from the list above, and it will be opened in the editor:
Once you have finished working with the diagram, click the Save & Exit button to save the changes and exit the editor. Alternatively, you can click Exit to discard any changes made and leave.
Afterwards, you will see a preview of the selected diagram.
