When you drill holes in the circuit board you want to drill all holes in the same set up to get the best position accuracy for the holes. The non-plated holes needs then to be blocked during the plating process (otherwise they will be plated as well). To be able to block the NPTH we need space between the hole edge and copper. The distance needed is related to the copper thickness. For standard copper thickness (18µm base copper), a good value is 0,3mm and definitely not less than 0,2mm.