You can assign units to your items. The unit is not used to calculate totals, only to modify the quantity, so assigning units is optional. But units can help you track items in terms of how you sell them. For example, suppose you sell staples by the box and by the case. You could set up two items for the staples: one for staples by the box and another for staples by the case. On an invoice, you could select the staples by the box and enter a quantity of 12, or you could select the item that corresponds to the case (which in this example is a case of 12 boxes) and enter a quantity of 1.
If you sell services, you might bill by the hour and the project. In this case, you could set up two items: one for your time by the hour with your price being your hourly rate and the unit being "hour," and another for projects with the price being "$0.00" so you can change it on the quote/invoice per project and the unit being "project."
When you create an item, you can enter the sales price of an item per unit (this is what you charge the people who buy it from you).
In additional, for inventory items, you can enter cost per unit (the amount it cost you to buy the item from a vendor). This way WorkingPoint can track your Cost of Sales.
How do I add an item?
How do I edit an item?