Features are added according only the "major.minor" part of the version number, the third part in the full version number is used for system target identification numbers for which the DLL was ported by Microsoft, and the last part is the build number on each target system version (which may change within regular system/software updates). Some hotfixes provide upgrades only for specific applications (notably in the Office installation directory), and not suited for use in the Windows system directory (whose version of the DLL should never be updated and is often protected by the system):