More than likely it is equivalent to sending an e-mail reservaton request. MS will try to book the dates for you if they are area available for your chosen resort and room type when they check the email. Email requests usually take a few days to process, you will then recaive a return email with the confirmation number for your vacation. If there is a problem with your requested dates, they will contact you via email, as well.
If I am booking a stay for a non-busy time and well within my 11-monh window, email is fine, otherwise I call.
Eventually, they are planning to take on-line reservations into real-time...but until they get online account management, it isn't practical, since borrowing or banking may be required.