In ZBrush, 3D models are called Tools as opposed to Documents, which are 2D or 2.5D images. The main difference is that tools are 3D models that can be rotated or scaled while working with them. Documents, as being images, are a flat representation with optional information stored in them.
Let's see how this can affect our workflow.
As you see, ZBrush's native format of saving 3D models(tools) is .Ztl.
ZBrush Documents are saved as