The DICOM Viewer is protected by two different types licenses:
The application creates a 3-day limited single license when started for the first time. The license is linked to the hardware signature of your PC. Remove limitations by registering the initial license. You will receive an Activation Key via email, an internet connection is required. This gives you a new 7-day license without limitations and enables upgrades.
The DICOM viewer licenses are available as a single license or as a network license.

When you have received a USB hardware key, there is need to register to use the software without limitations. First, start the DICOM viewer, then plug in the USB key. Wait for the pop-up asking you to use the USB key license. Confirm and restart the DICOM viewer. USB key drivers must be installed.

See also DICOM Viewer License explanation for more details.