Why the CString(LPCTSTR lpsz) constructor check the high two bytes of lpsz?

Question

I am reading the source code of CString in MFC. I am very curious about the implementation way of constructor CString::CString(LPCTSTR lpsz).

In my understanding, before copying the string indicated by lpsz, it only needs check whether lpsz is NULL but no need to combine with checking if HIWORD(lpsz) is NULL.

Is any MFC guy passing here and willing to give some explanations?

CString::CString(LPCTSTR lpsz)
{
    Init();
    if (lpsz != NULL && HIWORD(lpsz) == NULL)
    {
        UINT nID = LOWORD((DWORD)lpsz);
        if (!LoadString(nID))
            TRACE1("Warning: implicit LoadString(%u) failed\n", nID);
    }
    else
    {
        int nLen = SafeStrlen(lpsz);
        if (nLen != 0)
        {
            AllocBuffer(nLen);
            memcpy(m_pchData, lpsz, nLen*sizeof(TCHAR));
        }
    }
}