I looked into registry entry managing subsystem codes. Like native 1 GUI 2 CUI 3.
It is here HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\SubSystems\Windows
And it defines subsystem code 4.
Skipped here http://msdn.microsoft.com/en-us/library/ms680339(VS.85).aspx
It looks like special subsystem for SxS DLLs (yes, you are not blind). So I want to ask what makes difference between SxS DLL and normal DLL.