When you purchase a copy of Virtual Drumline, you are actually purchasing a license to use the sounds and software. Technically you do not "own" the software. You own the license to use it and make recordings of your compositions created with the software.
Per the included licensing agreement, Virtual Drumline is a single user license. This means you are permitted to install the software on two of your own computers. You may not share this license with other individuals. Each individual user must have his or her own serial number to own a license.
Software authorization is handled through the Native Instruments registration system which tracks your VDL serial number and assigns it an authorization code for the computer System IDs registered to your NI user account. This is all handled conveniently through the Service Center program which you install separately on your computer.
For more information on the Native Instruments registration system used with Virtual Drumline, click here.