Args
ARG(1) indicates which part of the window the mouse is over. It the same value as what you return from your NCHITTEST event handler.
ARG(2) is the X position where the mouse pointer is currently located (referenced from the upper left corner of the desktop screen), and ARG(3) is the Y position.
Notes
If the mouse is captured by some window, then the event happens only with the window capturing the mouse.
See also NCLBUTTONDOWN, NCRBUTTONDOWN, NCHITTEST.