Every plant has designated preferences towards 2 important metrics. pH value and TDS value. the pH value indicated the acidity of the nutrient solution while the tds value indicated the amount of nutrients. For plants in hydroponic systems it's very important that the pH value fits their needs because it helps them to absorb the nutrients from the solution. If the pH value is off, plants cannot get their nutrients and will die as a result. The TDS value indicates the amount of nutrients a plant needs. Fruits and Vegetables need more nutrients than herbs, moreove in warm months a plant needs more nutrients than in cold months. For the hydroponic system it's important to match the pH value of all plants growing in the system. Therefore the planner will always filter all the plants according to their fitness to the current systems pH value. Other than the pH value the TDS value of the system can be higher than the required value of the plant, but it should never be lower. As a rule of thumb, the TDS value of your system should be the max of all the lower threshold.