The Invoice Notes form is displayed when the Invoice Notes button is clicked (click Tools button in Toolbar menu to see the Invoice Notes button) on Invoices form. It enlarges the viewing area to see more of the notes written in the notes field. This can be very helpful if you have several paragraphs of notes entered in the notes field.
If this button is showing in place of the normal notes button then it means there are existing notes. This is just a visual way to tell that there are at least one or more notes entered and when clicked works the same way as the normal notes button does.