The thickness of your object layers is something you can choose. The 3D40, 3D40 Flex and 3D45 Printers have a maximum resolution of 50 microns (0.05mm), the 3D20 has max resolution of 100 microns (0.1mm). All printers have a minimum resolution of 300 microns (0.30mm). The higher the resolution, the smaller each layer and the better your object will look. The layer resolution depends on the setting:
Low - 0.3mm
Medium - 0.2mm
High - 0.1mm
Ultra - 0.05mm (3D40 and 3D45 only)