Here's what these terms mean in Bulk Discount Now:

Percentage: applies a percentage discount to your selected products
For example, a 5% percentage discount on a $100 product would give a discount of $5 off per unit for a price of $95 per unit.

Fixed amount: applies a fixed amount discount to your selected products
For example, a $5 fixed discount on a $100 product would give a price of $95/unit.
Fixed amount discounts are subtracted from every unit  not the cart total! For example, a $5 fixed discount on a $20 hat means each hat costs $15. If a shopper orders 4 hats, they'll pay $60 (4 * $15/hat) and NOT $75 (4 * $20/hat  $5 discount.)