HKEY_CLASSES_ROOT, often shortened as HKCR, is a registry hive in the Windows Registry and contains file extension association information, as well as programmatic identifier (ProgID), Class ID (CLSID), and Interface ID (IID) data.KEY_CLASSES_ROOT is a registry hive and so sits at the top level in Registry Editor 1  : Open Registry Editor. 2  Find HKEY_CLASSES_ROOT in the left area of Registry Editor.