SubTools are separate polygon objects. Each SubTool can be equal to the maximum number of polygons your system can handle. If your system handles 8 million polygons and you have 4 SubTools then your model can be composed of 32 million polygons.
SubTools can be hidden by turning off the eye icon next to their name in the list, or all but the selected SubTool can be hidden by using Solo mode.
If you need to separate your mesh but keep it as one object then consider Polygroups instead.
SubTools are a good way to construct a model that would naturally be made up of separate parts. So, for example, for a character model the body could be one SubTool and the clothes other SubTools. Using SubTools means that it is easy to work on one piece at a time and this makes best use of your computer’s resources.
Existing meshes can be added as SubTools by using the Append or Insert buttons which are in the Tool > SubTool sub-palette.
The Solo mode button is on the right of the standard interface and also in the Transform palette.
Solo mode isolates the selected SubTool by temporarily hiding all other SubTools in a single click, without the need to change the current visibility of all the SubTools in the SubTool menu. When Solo mode is turned off, all previously visible SubTools will become visible again while those that are hidden in the SubTool menu will remain hidden.
By default, Solo mode does not have an assigned hotkey. You can enable Solo mode’s single-click option by activating the Preferences >> Edit >> Click to Solo switch. When clicking in any empty part of the document you will isolate the selected SubTool and temporarily hide all others.
Speed up your productivity with the efficient new Folder System introduced in ZBrush 2019. Far more than an organizational tool, this feature also makes it possible to apply actions to all contained meshes at once such as Move, Scale, Rotate, Duplicate, Delete, Hide/Show PolyPaint, Live Boolean and more.
Read more about SubTool Folders here.