Memo is a type of reference that users can input manually. It is under "+ Reference" as it is considered a reference.
You can save your manual text inputs as reference material for your work. They can also be backlinked from other documents.
Click on "+ Reference" and select "Memo".
Input a title and input text, then click "Save".
It is now saved as a Memo which you can view and edit in the split-viewer as well as Backlink them if needed.