The unit cell is a two dimensional or three dimensional representation of the void network. There are three built-in viewers, as well as stand-alone viewers.
When you double click on a building operation, a fluid uptake operation or a diffusion operation in the operations list the following screen is displayed.
You will notice two help buttons 
on the right hand side of the page. The upper button takes you to a list of controls for the 3D full-screen viewer. It is a good idea to look at these controls before first use of the 3D full-screen viewer, because mouse instructions are not available once in full screen The lower help button takes you to a general description of the viewers.
Unit cell viewer choice screen