The optimized solution for creating a cell shading effect is to alter existing material curves as described above. This solution may make it somewhat time-consuming to first define a material, but it pays off later. ZBrush calculates this level of posterization as a pre-render effect, before even considering the colors present on the model (as PolyPaint or texture). This creates a truly 3D posterization which produces great results.
Global Posterization using Material Mixer
A global posterization can be applied to your materials by using the Material > Mixer sub-palette and increasing the Steps value found there.
This solution is different from the 3D Posterize found in the Render Properties as it is applied as a 2D effect such as you would find in a photo editing software. In other words, rather than being taken into account as part of the render, it is applied to the image after the render has been completed.
Yet it is also different from what you could accomplish in another program because as a Material Mixer, you can mix several shaders through a posterization value.
This is also where you will find the Outline slider. This gives that black outline to the model that you often see in comic books or cartoons.