In the case that not provider is given then whatever provider the box represents will be added to the system. Ideally, a provider will be given, but if not, Vagrant still does a "best effort" to install the box.
In the case that not provider is given then whatever provider the box represents will be added to the system. Ideally, a provider will be given, but if not, Vagrant still does a "best effort" to install the box.