Yes the nozzle is fixed such as 0.4. The idea is to make the change in the slicer to the nozzle size or the extrusion width to see if it makes a difference. If it does then you know that the area is to thin for the actual setup of the printer.
It is just a test to see if it is just to thin or if there is something else going on.
However, if you change the extrusion width to .39 or .38 for instance and it then shows that area of the part you can try to print like that. You can go a little lower than the nozzle size on extrusion width but not much.