Why need codepage in WideCharToMultiByte

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VC6

VC7.0

C++

Windows

Unicode

I have some confusion regarding WideCharToMultiByte function - we need to pass the codepage of the multibyte string in the first param of the function - Now my confusion is why this param is required? As the every unicode character is unique and there is no overlap so where this is required?

int WideCharToMultiByte(
  __in   UINT CodePage,
  __in   DWORD dwFlags,
  __in   LPCWSTR lpWideCharStr,
  __in   int cchWideChar,
  __out  LPSTR lpMultiByteStr,
  __in   int cbMultiByte,
  __in   LPCSTR lpDefaultChar,
  __out  LPBOOL lpUsedDefaultChar
);