Internal notes can be used on invoices to communicate between your team or with your accountant about special circumstances related to specific invoices.
You can find this function when you're creating an invoice, under Internal Notes.
To see internal notes on invoices, navigate to the Invoice portion of the Billing section.
Note: Internal notes are only available on Invoices, not Transactions.
Here you will click on Details and be taken to the Invoice Summary where you can see and Edit any internal notes.