Slabs are adspend (usage) ranges defined in the form of units like ($0-$500). Multiple price slabs are defined in the system for generating charges. In slab based pricing, based on the slabs, price is calculated and invoice is generated.
Simply, define ad spend ranges and set the slab for each range. So that, throughout the range, the same price would reflect. You can also set slabs like - the more the ad spend, the less the price.