If you check this the system will create system account for specified email address (normally this is your client, accountant, employee etc.) and generate and include special link in the email sent.<br /> Using the link the client can access not only this document you send now but also all other documents with the same partner.<br /> Using this way of sending document you can benefit of tracking exact moment when your client read the document. <br />If you dont check this the recipient will receive only pdf version or if you dont check pdf checkbox - only notification (and some basic information) for the existence of this document. <br />If option is disabled from administrator you cannot check it.