Discount type is the parameter that defines the discount as fixed or variable in terms of amount or percentage. Choose "Fixed" if you want a fixed discount for all your customers. E.g. $5 discount or 5% discount for all customers who show the scanned discount coupon at the cash counter.
Choose "Random" if you want variable discounts to be given to your customers. Eg: If the discount varies between $5 -$10, then customers will get discounts between $5 to $10.