TwitterCart is licensed on a per website basis. We know a lot of clients, webmasters, agencies and other businesses generally operate and maintain several wordpress websites.
If you fall into this category, then the Unlimited licensing makes more sense. Even if you wish to give a free install to your clients or a friend, with Unlimited you are able to install as many versions of TwitterCart you wish (subject to common sense abuse restrictions like posting your license on download websites etc.!)
Unlimited TwitterCart Installs. One Fee WordPress Licensing.