What is the HDC in VC6?

In the following functions:

Void CscrollView :: Ondraw (CDC * PDC)

{

HDC HDC1 = :: getdc (getsafehwnd ()); HDC HDC2 = PDC-> getSafeHDC (); hwnd hwnd = m_hwnd; //}

What is the difference between HDC1 and HDC2;

I use HDC2 to draw a drawing operation, the operation scroll bar can be automatically moved, but DPTOLP is wrong, the conversion result is always positive;

I use HDC1 to draw a drawing operation, and the operation scroll bar cannot be automatically moved, but the dptolp is correct;

I see MSDN, I haven't seen anything.