Qty Pricing lets you set pricing based on quantity, such as lower price for buying 100. Turn on this feature in Item Prefs by clicking Use Qty Pricing to add a Qty Pricing tab.
Qty Group lets you group items to use their combined quantity to choose Qty Pricing. An invoice for grouped items with Qty 70 and 30 would use 100 Qty Pricing for both.
Once you enable Qty Pricing, choose Show List in the Qty Group dropdown to create. Then use Update on the Items Search to set multiple Items to use the new Qty Group.