A single trade discount is one discount offered, such as 30%.

A discount series will be a number of discounts offered in a chain. It is shown by slashes - such as 15/10/5.

You'll notice that the 15/10/5 numbers add to 30%, but it's not the same as a 30% discount. I'll show you how to compute it, and I think that will make it clear.

$100 item at 30% off means you save $30 - it costs $70.

$100 item at 15/10/5. First take 15% off the $100 = $85. Then take 10% off the $85 = $76.50. Then take 5% off the $76.50 = $72.68 (rounded).