If the message is WM_MOUSEWHEEL, the high-order word of this member is the wheel delta. The low-order word is reserved. A positive value indicates that the wheel was rotated forward, away from the user; a negative value indicates that the wheel was rotated backward, toward the user. One wheel click is defined as WHEEL_DELTA, which is 120. If the message is WM_XBUTTONDOWN, WM_XBUTTONUP, WM_XBUTTONDBLCLK, WM_NCXBUTTONDOWN, WM_NCXBUTTONUP, or WM_NCXBUTTONDBLCLK, the high-order word specifies which X button was pressed or released, and the low-order word is reserved. This value can be one or more of the following values. Otherwise, mouseData is not used. XBUTTON1 The first X button was pressed or released. XBUTTON2 The second X button was pressed or released.