If the user moved the mouse, which is indicated by a value of dwFlags that includes MOUSEEVENTF_MOVE, dx and dy hold information about that motion. The information is specified as absolute or relative integer values.
If MOUSEEVENTF_ABSOLUTE value is specified, dx and dy contain normalized absolute coordinates between 0 and 65,535. The event procedure maps these coordinates onto the display surface. Coordinate (0,0) maps onto the upper-left corner of the display surface, and (65535,65535) maps onto the lower-right corner.
If the MOUSEEVENTF_ABSOLUTE value is not specified, dx and dy specify relative motions from the position reported when the last mouse event was generated. Positive values mean that the user moved the mouse right or down; negative values mean that the user moved the mouse left or up.
Relative mouse motion is subject to the settings for mouse speed and acceleration level. An end user sets these values using the Mouse control panel application. An application obtains and sets these values by using the SystemParametersInfo function.
The system applies two tests to the specified relative mouse motion when applying acceleration. If the specified distance along either the x or y axis is greater than the first mouse threshold value, and the mouse acceleration level is not zero, the operating system doubles the distance. If the specified distance along either the x or y axis is greater than the second mouse threshold value, and the mouse acceleration level is equal to two, the operating system doubles the distance that resulted from applying the first threshold test. It is thus possible for the operating system to multiply relatively-specified mouse motion along the x or y axis by up to four times.
Once acceleration has been applied, the system scales the resultant value by the desired mouse speed. Mouse speed can range from 1 (slowest) to 20 (fastest) and represents how much the pointer moves based on the distance the mouse moves. The default value is 10, which results in no additional modification to the mouse motion.