When you go to the details page of an invoice, you have the possibility to add comments.
If you want to share these comments with your accountant, you can attach them to the PDF invoice.
To set this up, go to your Settings > Incoming > Share with accountant.
From here you can choose to include the comments on the first or last page of the invoice's PDF.
Then you can choose to include (or not) the attachments from an invoice's details page.
Of course, you can continue to use comments without sharing them with your account, simply uncheck the Append comment option.
You can always change whether or not you want to share a comment, as long as the invoice has not been archived yet:
Note: any comment you want to share with your accountant must be added before the invoice gets archived (i.e. before accepting a purchase invoice or before sending the customer a sales invoice). Comments added after archiving will always remain internal, therefore not visible to the accountant.