A low threshold value makes it possible to select the Soft tissue of the scanned patient. With a high threshold, only the very dense parts remain selected. Using both an upper and a lower threshold is needed when the nerve channel needs to be selected. Defining a good threshold value also depends on the purpose of the model. If you just want a nice looking model, a lower threshold value is recommended since it will result in a model with fewer holes. On the other hand, when the model serves for modeling prostheses a higher threshold value is preferred.