HSDS Modifier
The HSDS modifier is useful for finishing up your 3D models. It allows you to add details to low polygon areas of your model.
using local refinement lets you subdivide sections of your mesh and edit the mesh in that area. This is automatically done by altering sub-objects on a grid. You should use this if you need to increase the mesh resolution in certain areas rather than subdividing the entire object, this is much more efficent as you create polygons where you need the detail, as opposed to the tessellate modifier. A use of this function would be for a human hand. Once the basic shape is modelled, the modifier can be used to add bumps for the knuckles.
Many levels of detail can be added using the HSDS modifier, because of its hierarchical nature. Subdivision Stacking allows you to navigate these various detail levels any time while you are using the modifier. This means you can edit any part of a mesh at any mesh resolution. If you work at a lower detail level than the highest one, the high detail areas will still be in effect, but you can control them indirectly by altering the lower levels which have a higher influence due to their position in the heirachy.
You can animate parts of the modifer only at the lowest detail level. This is done by animating the objects base level on the stack, i.e below the HSDS modifier. To animate the mesh after HSDS modeling, you must collapse the stack.