Besides the project notes, you can also create notes for invoices, payments, etc. You can enter different types or categories of notes using the Project Notes screen.
To create an invoice-related note:
Open Invoice Review screen from the Billing menu.
Add the Notes column to the grid by selecting Options and then Field Chooser.
Use the filters to select an invoice for which you have to record a note and click Refresh.
Click the Notes link for the invoice record.
In the Project Notes screen, the ‘Invoice Review’ Category is pre-selected. Enter your Note.
Click Attachments to link any file to the notes.
Click Save and then Return.