With this menu you can compare 2 different render results in LV. For example you want to change the color of a material, but also compare it with the current appearance. In this case, select "Store Render Buffer" from the menu. As soon as you select this option, the "Enable A / B comparison" in the same menu also becomes active. Now you can make the changes and see the result in LV window. You can also see the difference between the stored and the current frame by dragging left or right the A / B line in the LV window. to close the Compare, select "Enable A / B comparison" again from the menu