As stated in the instructions accompanying the key, the permissions and ownership of NX Server 3.5.0 subscription files should look like the following:

-r--------  1 root root 703 Jun 26  2006 /usr/NX/etc/node.lic
-r--------  1 nx    root 711 Jun 26  2006 /usr/NX/etc/server.lic

The NX server will check that the file has the correct permissions at run-time, for example before running a new session.

The reason why the file has to be readable only by the NX user (the user that is impersonated at run-time by the NX server) is that the license key contains sensitive data that is used by the customer to prove the actual purchase of the software and get access to the NoMachine support. A NX Server machine is likely to be made available to multiple concurrent users, from inside and outside the company. If the key was not protected, any user having the credentials to run a session would also be able to gather sensitive information from the license key of the organization offering the service.