European versions of Claris products use Network serialization (this serial number has nothing to do with the Registration number).
If the same copy is on two different machines in the network then only one can be launched at a time. Attempting to launch the second will give an error message saying that the maximum number of users of this application has been exceeded.
Claris also offers a Concurrent User License (CUL), which does allow the same copy to be launched multiple times but counts the number of launches and when a preset limit is reached it will disallow and further attempts to launch.