log in as user1, activate with prodkey1, on LimeLM website it shows that prodkey1 has one activation. log in as user2, shows that user2 is not activated, TurboActivate allows user2 to activate with prodkey1 and on LimeLM website it shows that prodkey1 still has only one activation.
Using either TA_SYSTEM or TA_USER the activation is for the system, not the user. The only difference between TA_USER and TA_SYSTEM is when the license files are stored.
If you want to do per-user-session licensing you'll have to use TurboFloat (a floating license server, coming soon).
Does that make sense?