Let's assume you are in a position where you have sent a a lead buyer a masked certificate. Let's also assume that later on that year, you need to send them the full certificate. It is possible to send them the full certificate if you are the full certificate holder. If you hold the full certificate in your account, and view a masked certificate (while logged into your account), it will reveal the corresponding full certificate.
For a full overview of the concept of 'Masked Certificates' click here.