This feature allows you to use a price, either cost or selling, that is per a number of items.
For example you could list one of your selling prices as being 45.67 per 10. This Per filed also filters down to Customer and Supplier Invoices etc.
You can also use the Per field where the price is to more than three decimal places. For example a Selling Price of .35 per 100 equates to a Selling Price of 0.0035