Debtor notes created in the open invoice are set as 'private' and cannot be seen in the project portal on the shared invoice.
New debtor notes can be added in the open invoice view to save internal comments about the invoice payments.
To create a new debtor note for an invoice:
Open the invoice using the organization invoices list, or the project invoices tab.
In an open invoice scroll to the bottom of the finalized invoice to find the 'debtor notes' section.
Click the 'Add' button in the top right corner.
Type a short subject line to be a summary of the note.
Type the note description into the 'note' box, to record the details of the phone call or task.
Save.