single trade discount is a one time discount while a discount series can't be added together but have to be discounted separately wherein the base for the next discount will be the net of the first discount and so on.

a seller can offer a series of discounts on a product for a number of reasons like for example, 20% discount upon sale, additional of 5% if volume sale and another discount of 5% if the account is paid within 30 days.

the buyer gets a higher discount if it is given as a single discount rather than a discount series. for example a single discount of 30% compared to a discount series of 20;5;5. if the sale was $ 100 the discount is $ 30 while if the discount is series. the discount is $ 27.80

A single trade discount is one discount like 30% where as a series discount is more then one like 20/10/5 but can not be added together, have to be discounted separately.