If you are using physically held shares in a company as collateral for a long CFD position in the same stock, the positions will be aggregated when calculating the margin tiers. This means that the physical and CFD positions will be considered holistically to determine your total position size and how this fits into the defined tiers.
Conversely, if you open short margin positions in the same stock, the margin and physical positions will be tiered separately, with no offsetting between the two.