USB Servers Statuses.
Each computer has its own status. Computers' statuses depend on whether USB device is located in the Window of USB devices available for Window of USB devices available for connecting.
There are three kinds of icons:
this icon means that RDP session is established;
this icon means that RDP session can not be established for some reason;

Connection fails in the following cases:
USB Redirector RDP Edition versions are different;
USB Redirector RDP Edition is not running on the remote PC.