DPSubPix

From ZBrush Info

The DPSubPix value specifies how many times a mesh will be subdivided internally in order to generate the displacement map. So a value of 4 means that the mesh will be subdivided four more times, increasing the number of polygons to 256 times as many. In other words, if the mesh contains 2 million polygons, then a DPSubPix of 4 means that ZBrush will need to calculate a mesh with 512 million polygons in order to create your map!

While working on a model, a default DPSubPix value should be used. Only increase the value when you are ready to produce the final map. Don't increase it needlessly, however -- there is certainly a point of diminishing returns.

Here's how to evaluate the sup pix:

Let's assume that you have a mesh with 1 million polygons and you are generating a displacement map of 2048x2048. The map contains 4 million pixels, so if you use a DPSubPix of 1 it will mean that ZBrush will calculate 4 million polygons. This results in a 1 to 1 ratio of pixels to polygons. If the map is 4096x4096 then you may want to use a DPSubPix value of 2 in order to achieve the 1 to 1 ratio. Using a value of 4 in such a case would be overkill.

This is true assuming that the UV mapping evenly distributed across the map (such as when using AUVTiles or GUVTiles). On the other hand, if you've generated different UV mapping and have an area in which a small number of polygons are mapped to a larger map area you may need to increase the DpSubPix in order to achieve a good ratio of polygons to pixels in that particular area.

If you do need high subpix in one area and lower in another, you may want to generate the map in several steps. Since ZBrush is generating the map for only the visible portions of the mesh, you can hide a portion of the mesh and use the appropriate DPSubPix value for that area. You can then reveal a different portion of the mesh and adjust the DPSubPix accordingly. When done, merge the multiple maps into one. This method requires careful planning, but can save a lot of time compared to using a high DPSubPix value for the entire mesh.

DPSubPix is located in the Displacement sub-menu

Learn more about Displacement Maps at Wikipedia.