Valve Software's proprietary file system changes from .SIM/.SID to .CSM/.CSD
 

Valve Software, owners of the Steam digital distribution platform have always used a proprietary file system to deliver their content. This was done for command and control reasons and to try and prevent hacking of their content. The original file system format used for several years, ended in the .SIM and .SID extensions. The new format --switched to recently-- ends in .CSM and.CSD file extensions. This applies to both retail discs and download depots.