This message requests table window to scroll up or down lParam bytes. wParam contains number of completely visible lines in the window (1 if data area is smaller than 1 line).
Standard table window should simply pass this message to Tablefunction where it is interpreted as WM_USER_VABS.
Custom table window must modify data but neither redraw nor invalidate the window. If position of data remains unchanged, window's function must return -1. If window supports byte scrolling, it must return (topmost line)*MAXTRACK/(total number of lines). If total number of lines is less than or equal to wParam, it returns 0. Otherwise, it must return (topmost line)*MAXTRACK/(total number of lines-wParam). As constant MAXTRACK is relatively big, use MulDiv to calculate return value.