The shells of a model refers to the wall as well as the top and bottom of the object. The only parameters that should be changed are the wall thickness and top/bottom thickness in this menu. The shell thickness is the most important part in changing the strength of the printed part.
Make sure that the wall thickness setting is increased in increments of the nozzle diameter, which in this case will always be 0.4mm. The top and bottom thickness should be set in increments of the layer height, which is adjustable as previously shown. The diagram below details the parts of a print relevant to this portion of the guide. Additional information can be found here.
