Auto-Complete Block

After drawing a vertically extruded block perimeter on a horizontal plane, the block will be automatically extruded by the Height setting if the Auto-Complete Block option is switched on or the  View Rotation has been set to 'Plan'. With this option switched off (or in the case of a non-horizontal drawing plane), the perimeter can be dragged along an axis normal to the perimeter to obtain the required extrusion.

See also the Creating a model Tutorial