Data is drawn in layer order, layer 1, then layer 2, then layer 3, so layer 3 is 
covering layer 2 which is covering layer 1. 
However, this order is only true on a per cell basis. 

So if you have a TOP cell which references CELL_A and CELL_B, 
then it will draw all data in TOP in layer order, then data in CELL_A in layer order, 
then data in CELL_B in layer order. 

This means layer 1 in CELL_A will cover layer 3 in TOP.