When you buy an MSDN Premium (or formerly Universal) subscription, you get development licenses for a wide range of MS servers. These server licenses are only good for development and testing purposes, not for production use. The idea is that if you are developing a solution using these servers, you don't need to run out and purchase a full blown license for the server. This is a great idea to help keep costs down and promote MS Server based solutions.
The exception to the above is the Team Foundation Server. With the Premium subscription, you get a license to use a limited 5 user version of the server (Note: the limitation is on the number of users, not the functionality of the server) for what I would argue is production use. In other words, you can use the Version Control, Work Items and other features as if you bought the full server (as long as you stay within the 5 user limit). This is significantly different than the licensing for all of the other servers included with the Premium subscription.
So, what do you do if you want a server to develop and test the customisations that you are doing to your full-blown production TFS server? Well, if you have more than 5 developers or you want to test a solution with more than 5 users (including performing load testing), you need to buy a copy of the retail TFS server! That's right, no special development server license is available, you need to either use the 5 user edition or buy a full server license. Granted, the server isn't that expensive and you should already have the client licenses (Team Foundation Explorer), but it really would be nice to have a development licensed server.
Microsoft, are you listening?