CPA stands for Cost per Action or Acquisition. In this campaign, you have to pay us (publishers) after someone purchased your product or service. This is a low-risk way for advertisers to buy media. And a high-risk way for publishers, as they have to run advertising with risk. If no one buys the service, the publisher will not make money. So, the banner is best for advertising mode but worst for rate.
CPA = Cost to the Advertiser / Number of Conversions