These notes can be chosen to be for internal purposes or they can be displayed on the invoice. That is decided in the invoice builder.

To add or view a note, you can go into an order and click on the Manage tab on the workflow bar.

To create a new note, click on the little "+" icon on the top right side of the drop down. There is no character limit for these order notes and you can also upload files (png, jpeg, word, csv, xls, and pdf).  

If you click on the little flag icon (red arrow) in the note, it will flag it as import to be seen on the Business Dashboard.
If you want to edit the existing note or delete the note, you can click on the carrot (blue box) to see the options.