Windows Server 2008 Build 6003 refers specifically to the version of the operating system after the installation of Service Pack 2 (SP2)
When a server is updated to build 6003, the change is visible through standard system tools provided by Microsoft Support About Windows Dialog : The version will show as 6.0 (Build 6003: Service Pack 2) windows server 2008 build 6003 upd
If you are still running Build 6003, your system is likely vulnerable to newer security threats. Windows Server 2008 Build 6003 refers specifically to
Compatibility: Most users noticed no difference, but scripts or applications hardcoded to check for build "6002" required updates to recognize "6003" as the same OS. Support Status windows server 2008 build 6003 upd
If you maintain a 2008 server, verifying that it is on build 6003 (not 6002) is the best indicator that it has received all possible ESU security patches.