A user CAL is assign to one user. A Document CAL is assigned to a document. This in the pass was called Q4C, and was designed so you could have a high number of users assigned to one document on a QlikView server. So, in the past this would require companies to have two servers. One for Q4C and the other for normal users. This was changed with the introduction on document Cals. Now you can have both on the same server.