The NDL is designed to keep your tissues from going into a state of nitrogen saturation from the compressed air you are breathing. If a diver goes over the NDL, tissues are saturated. A series of decompression stops are required to release the nitrogen through breathing to limit the possibility of decompression sickness. For this training program, going over the NDL will be avoided because this requires more time and more breathing gas, and it is for divers trained to a much higher level in decompression diving. It is much safer to be able to ascend without having to do decompression stops.
Because NDLs are based on depth and time that a diver spends underwater, the deeper you dive and the longer you spend at depth, the more nitrogen we will absorb. To stay within your limits, you can calculate the safe times using dive tables or for more accurate planning you can use a PDC. Both use mathematical algorithms to determine the safe limits of your dives.