Hello ~viper~
Please note that 2011 and 2013 version work only on Windows.
To answer your question Infiltration CE 2011 is unofficial version published by soultaker007
some time before he released Infiltration CE 2013.
Sorry for bumping up the topic again...
Please read carefully
By the way there are 3 active Infiltration multiplayer servers (2013-07-12):
Infiltration CE (published on official Infiltration site) is needed to join this one:
Inf 2.9 CE server: (there are bots on the server)
75.178.9.14:7777
Infiltration CE 2011 (published by soultaker007) is needed to join these:
XploD2 Co-Op (you also must have Unreal Gold singleplayer maps installed)
81.169.152.68:8888
- XploD TDM (there are no bots on the server)
81.169.152.68:7777
Please note that 2011 and 2013 version work only on Windows.
To answer your question Infiltration CE 2011 is unofficial version published by soultaker007
some time before he released Infiltration CE 2013.