This is part of Win16 API which allow to create versions of program from one source code to run under OS/2 and Win16. Under OS/2 program can be running under Win-OS/2 if program is Windows NE executable, and with help on Windows Libraries for OS/2, if it is OS/2 NE executable. Here is a WLO to OS/2 API mapping draft
Releases the handle of the key specified by the hkey parameter by closing the key. The Registration Database is updated when all keys are closed.
LONG RegCloseKey (HKEY hkey);
Key handle
If successful, the function returns ERROR_SUCCESS. Otherwise, it returns an error value.