The VDA is installed on each physical or virtual machine in your site that you make available to users. Those machines deliver applications or desktops. The VDA enables the machine to register with the Controller, which in turn allows the machine and the resources it is hosting to be made available to users. VDAs establish and manage the connection between the machine and the user device. VDAs also verify that a Citrix license is available for the user or session, and apply policies that are configured for the session.