You're viewing the documentation for the experimental version of Vagrant (the 1.x series). The current stable series is 1.0.x and the documentation can be found here. You can learn more about the Vagrant versioning scheme here.
Configuration key: config.vm.box_url
Default value: nil
This points to a valid URL to the box that the VM requires. This is optional and is only used if the box doesn’t already exist on the user’s system. If vagrant up is called and the box is not found, this value is used to download the box.