The screen percentage coordinate system is useful for commands where you speak the coordinates of a pointer position or offset. It has (0,0) at the upper left corner of the screen and (100,100) at the lower right corner of the screen.

This function drags the pointer from the current position to a new position, dx screen percentage units away horizontally and dy away vertically.

CopyResize window vertically

Window (Top | Bottom) 1..99 (Up='-' |  Down='+')
    = MoveToEdge($1) DragByScreenPercent(0, $3$2);

Saying for example "Window Bottom 3 Up" resizes the current window by dragging the bottom edge up 3 screen percentage units. $1 is "Bottom", so MoveToEdge(Bottom) moves the pointer to the bottom edge of the current window. $3 is "-" and $2 is "3", so DragByScreenPercent(0, -3) drags the pointer upwards 3 screen percentage units.