In previous versions where QlikView Server was not available, you had to have your application registered with your serial number and so. If you changed your computer, or it crashed, you had to contact the support team in order to clean the license number to make sure you are using only one computer with one license.
With the introduction of QlikView Server, you only have one license number (one PDF) that allows the server to lease licenses to users, so when a user with an assigned license connects can access all documents and besides, can work offline with the application fully licensed fro 30 days. So it's the QlikView Server that control whether a user has or has not a license.
This environment is getting more and more usual every day, so you don't need to have your copy of QlikView Desktop (the software you run to develop QlikView documents) registered and licensed. Instead, you work in a server environment that allows you to work licensed for 30 days when you get offline. Everytime you go online and connect to the server, the lease is updated to another 30 days, so if you always work connected to the server (for example, in your office) you are virtually always licensed, although the license number is not visible in your Desktop application but in the Server itself.
Hope that makes sense.