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 campaign response and pass further to the publisher.
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.