The login context - accessible with the do.ashx?cmd=who command and from within XSLT in the $context parameter - contains a 'SystemGuid' attribute that uniquely identifies the GN4/GNPortal/Tark4 system being used:

  <LoginContext xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" 

    Guid="9c8f0133-8558-4311-b343-0f9a66d7049f" 

    UserId="obj1705" UserName="TeraDP\MiMo" UserFullName="TeraDP\MiMo" UserEMail=""   

    ServerAppName="gnpc" ClientAddress="::1" ClientAppName="WebBrowser" ServerUrl="http://localhost/GN4" 

    ServerVersion="1.2.960.0" ClientTimeZoneOffset="01:00:00" UserLockLevel="1" 

    SystemGuid="15a64d6b-5c31-48b0-838b-0d62f7904121" xmlns="http://www.teradp.com/schemas/GN4/1/LoginContext.xsd">

    . . . 

  </LoginContext>

This information can be useful when exporting data to uniquely identify where the data comes from and generate unique identifiers - combining the SystemGuid with the object ids.