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As many guessed this CBT5 is all about stress testing of servers and client.
First thing that they are testing is optimisation of servers and client as well as start and middle content. On the first day (not the first day when Korean players from internet cafes were allowed to the test but second day when everyone were allowed) firt server crashed with 50,000 players online. Server was successfully up in short time.
XLGames wasn't embarrassed and told us that after server crash they spend 5-6 hours to fix the server. On the second day everything went much faster.
Also was made optimisation of 64-bit client and all players now can see improvement in fps and work is not finished yet. They wont forget 32-bit client as well...
Full article is here.
Information that this test is to improve work of the servers and client before release of the game was proved in interview with J. Song. Also he answered some questions about content (maybe later there will be translation of this Interview).
Link to the interview