Straight from the Toontrack website:
"Toontrack uses a distribution system called VSSD (Virtual Stock Software Distribution). This means that we ship all products installer discs to the customer collated in a single box. The advantage of the VSSD system is that you won't have to wait for the discs to ship if you wish to purchase another Toontrack product, as you will just need to buy a serial number online.
You will however only be able to authorize and use the product(s) you have actually purchased and have registered a serial number for to your Toontrack user account.
The most common scenario causing confusion is when a customer buys a product (most often Superior Drummer 2.0 or any SDX), then receives the VSSD box and installs products that he/she does not have serial numbers for.
Again, a registered serial number is required to complete the authorization process, and you will not be able to authorize and use the installed products other than the one(s) he has registered to the Toontrack account."