HKEY_CLASSES_ROOT (HKCR) Windows registry key contains file name extension associations and COM class registration information such as ProgIDs, CLSIDs, and IIDs and is primarily intended for compatibility with the registry in 16-bit Windows.

HKEY_CLASSES_ROOT, Windows 2000 and above, is a compilation of user-based HKCU\Software\Classes and machine-based HKLM\Software\Classes. If a given value exists in both of the subkeys above, the one in HKCU\Software\Classes takes precedence.[8] The design allows for either machine- or user-specific registration of COM objects.

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