PAR Levels and Reorder Quantities can be thought of as a floor and a ceiling respectively.
PAR Level - This is the ideal minimum quantity you want in stock at all times. This is similar to Alert level in Shopventory. This is valuable for keeping a well-stocked shelf and avoiding running out of product.
Reorder Quantity - This is the quantity you want to get back to when the quantity hits the PAR Level. This is your ideal maximum for the given product.
For example, let's say I want to keep a minimum (PAR) of 10 units on my shelf at all times. Once I get to 10 or below, I want to get back to a (Reorder) quantity of 20.
So if I were to get down to a quantity of 5 units (below my PAR of 10), my next Purchase Order would automatically order 15, to get me back to my Re-order quantity of 20.