This box is a drop down menu that displays a list of pre-defined units that can be used, alone or in combination, to define the input or output units for a problem. Use the drop-down arrow to see the list, click on a unit to see its description appear in the unit descriptions box to the right. Since the predefined units can be combined in equations to represent complex combinations of units, the list is fairly short. Of course, new units can be added to the list at any time.
The image below is clickable... click on any feature to see an explanation of its use.