Message requests table window to start selection. HIWORD(wParam) contains column where selection begins, LOWORD(wParam) - X offset within the column in character widths, lParam - Y offset within the window in character heigths.
Standard table window should simply pass this message to Tablefunction.
Custom table window must modify data to reflect start of selection but neither redraw nor invalidate the window. It must return 1 if screen appearance is changed, 0 if not and -1 if start of selection at this point is not possible.