This field is used to describe the units selected in the concentration field. The Per field is comprised of two parts: the left field is the number part of the substance per and the right dropdown is the substance per units. To enter a substance per, put the dose number associated with the concentration field and choose the units from the right dropdown. You can either click the down arrow to open the dropdown or type the first letter of the unit you want to use until the one you expect appears in the window. You can also type the units with the number part and it will transfer the units to the units box for you.