Before returning a handle that identifies that DC, the window manager selects a drawing surface into the DC. If the application called the CreateDC function to create a device context for a VGA display, the dimensions of this drawing surface are 640 by 480 pixels. If the application called the GetDC function, the dimensions reflect the size of the client area.
When an application passes the handle returned by CreateDC or GetDC to one of the graphics device interface (GDI) drawing functions, the requested output appears on the drawing surface selected into the device context.