Confirmed - there is not way to just update the virus definitions without using a license.
It is a calendar month, e.g. if you use on 3 different PCs on April 25, you can use on 3 new PCs on May 1.
Some people can put this on a normal USB. Other people prefer to buy a FixMeStick that is designed to work on the broadest range of PC firmware, supports WiFi, updates the software and definitions and stores them on the device, uses multiple AV engines to maximize detections, and is supported by a friendly technical staff and a satisfaction guarantee ;)