Bid Optimization Rate (can be called revshare on campaign) has been made in order to let you set the % not only on the Supply side, but on Demand as well. We subtract mentioned percentage on the campaign level from the bid in RTB / Prebid campaign response and pass further to the publisher.
You can find this field in the RTB / Prebid Campaign Basic settings.
For example, you have set 30% as Bid Optimization Rate. It means that we will subtract this 30% from the bid in response and pass this response further to publisher:
5$ came as bid in the response, we subtract 30%: 5$-30%=3,5$ and pass this 3,5$ back to the publisher and recorded as a Total Revenue.
NotePlease keep in mind that Bid Optimization Rate and margin on the Supply side work in pair.