One thing you may have noticed is that you're only able to issue invoices to your Clients. But what if, whether the invoice concerns one of your clients or not, you want some third party to be able to receive and pay it?
When sending an invoice, CA will NOT send a magic link to a 3rd party, for that sort of magic link will work to automatically log client in as that client to do the payment.
Instead, to send invoices to a 3rd party, that would be the stuff of issuing an invoice to a Company, for which you have a designated Personnel to be the recipient.
You'll enable them on your My Account >> Add Ons page, then create the Company, then the Personnel which is to receive and pay the invoice (being certain to indicate that person as able to receive and pay company invoices).
Once set up, you'll be able to issue the invoice to the Company.