From ZBrush Info

ZBrush allows you to subdivide to create additional geometry in specified areas of a model, a process known as local subdivision. This is in contrast with standard subdivision, which subdivides the model as a whole.

A major advantage of local subdivision is that it can add resolution in areas where it is needed, while adding relatively few polygons to the entire model's total. In contrast, subdividing an entire model quadruples the number of polygons.

The most significant disadvantage to local subdivision is that it alters topology in the areas being subdivided.

Subdividing an Area

To subdivide an area of a model:

1. Mask the model, leaving unmasked only the area you want subdivided.
2. Set Tool:Geometry:SDiv (subdivision display level) to 1.
3. Press the Tool:Geometry:Divide button (hotkey Ctrl-D). Additional presses will result in further subdivisions in the unmasked areas.

Local Subdivision Example

Here's a simple example of a typical situation. We have a model which will need more facial detail than originally anticipated, and we want to add resolution to the facial areas, without making the rest of the model more heavy-weight.

We start with the basic model (most of the body not shown).

Mask out the areas we do not wish subdivided; in this case the neck, back of the head, and the rest of the body (not shown).

Pressing Tool:Geometry:Divide (hotkey Ctrl-D) subdivides only the unmasked area. The geometry between the subdivided and unsubdivided areas is adjusted for a smooth transition.

Here's a closeup of the boundary. When doing a local subdivision, the three resulting areas--the fully subdivided area, the boundary area, and the unsubdivided area--are assigned to different polygroups, making it easy to hide and work with them afterward.