Depending on what ad type you’re creating, you can select between different optimization goals and billing events. If you create a Link ad, you could for example select the 'Offsite conversion' Optimization goal, pay for impressions and let Facebook bid for you by using the 'Lowest cost' Bid strategy:
From this section you can choose what you would like Facebook to optimize for. You’ll be able to select between different optimization goals depending on what ad type you are creating.
Billing event (Pay for)
The billing event decides how you pay for traffic. Your options in this section will change depending on your optimization goal.
You can choose between 'Lowest cost', 'Lowest cost with cap' and 'Target cost'. If you choose 'Lowest cost', Facebook will adjust the bids to try and give you as many optimization goals as possible (at the lowest cost) based on your budget.
You can learn more about Bid strategies in the article below: