When using cost-per-acquisition (CPA) bidding, an advertiser bids using a maximum CPA and pays by:
Explanation: Target CPA bidding uses your conversion tracking data to avoid unprofitable clicks and get more conversions at a lower cost. Based on your campaign’s history of conversions, Target CPA bidding automatically finds the optimal cost-per-click (CPC) bid for your ad each time it’s eligible to appear. It sets higher CPC bids for more valuable clicks and lower CPC bids for less valuable clicks.
Read more here: https://support.google.com/adwords/answer/2390684