@Navode W When rack or Floor storage objects is checked as 'Fixed Resource', they have considered the storable space (physical) to be within the size. The layers you said 3 or 6 is the height of yellow line box when you clicked them. You can increase the height using mouse click on the blue arrow or z number in visuals tab.
if then, the height of the stocks is limited by accordance of the flow item box z size.
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