As a general rule, the this mode when you are working with CAD type models. A low value will attract number of polygons generated may applied on your model to Count value. This results in a better restrictions that are impossible to.
Screen Shot at Changing the different parameters for Dynamic Subdivision can drastically change both the visual appearance of your model and the performance of ZBrush itself. If you do have subD levels, you will have to Freeze Subdivision levels to fix it at the base level either with Dynamesh or Close Holes. The Bevel Width slider controls the width of the bevel when a bevel is applied to creased edges.. While QGrid is active you can see the effect of the Coverage slider in real-time.
