Polygroups allow you to organize a single mesh with visual grouping information. Press SHIFT + F or press the Draw Polyframe button as shown selected in orange below to see the polygroups. Notice that the example image has six different polygroups as depicted with different colors.

Normally, polygroups display sections of a single mesh. If you want to convert polygroups into SubTools you can do the following.

If your model has different elements go to Tool > Polygroups and select Auto Groups. You can then go to SubTools > Split and click Groups Split. Groups Split will be disabled unless all polygroups are visible on the canvas (press CTRL + SHIFT and click an empty area of the canvas to display all polygroups).

When you want to merge all the stuff again just make each SubTool visible and pick SubTool > Merge > Merge Visible. ZBrush will create a new tool with all the former SubTools together again. Note that in merge visible you have an option for weld verts.

Warning: When you merge visible SubTools you will lose sublevels and the merged new tool is created with every SubTool at the current level. (At this moment, I don’t really know what this means, but it seemed like a relevant note, so I’m keeping it until I find out and can clarify the statement.)

In the brush palette, you can go to Auto Masking and set Mask By Polygroups to 100. This will setup an automatic masking so that when you use a brush, it will only affect the polygroup.

To try this, press CTRL and paint a mask on an object.

Press CTRL + W to turn the masked area into a polygroup.

Select the Move brush ( B + M + V )

In the Brush palette, under Auto Masking, set Mask By Polygroups to 100

Now, when you use the move brush, it will only move the polygroup because everything else is auto-masked.

Suppose you have made a polygroup on one side of your subtool (using the SliceCurve brush, for example)…

If you want to mirror that polygroup onto the other side of the subtool, follow this procedure.

In the Deformation palette, on the Mirror button, select the axis on which to mirror (usually x).

Click the Mirror button. You’ll see the polygroup flip to the other side.

Now go to the Geometry palette, expand the Modify Topology section, and click the Mirror and Weld button.

The polygroup is then mirrored to the other side as shown below.