Whether it is background or foreground you draw on same DC. This is only an option given to you to separate background and foreground. The usefulness comes when 'InvalidateRect' etc. are called in the program. Normally, WM_ERASEBKGND and WM_PAINT both get called ('resize' say). If you use InvalidateRect(...,...,false) only WM_PAINT gets called. If you use InvalidateRect(...,...,true) then WM_ERASEBKGND gets called when you use 'BeginPaint' inside WM_PAINT.

If you use InvalidateRect(...,..., false) then only WM_PAINT is sent. If you use InvalidateRect(...,..., true) then WM_PAINT is called and when you use 'BeginPaint' inside WM_PAINT it calls WM_ERASEBKGND (when WM_ERASEBKGND is called, the Invalid rectangle is nullified). In a normal circumstance the Invalidate rectangles received in WM_ERASEBKGND and WM_PAINT are same.