Campaign goal, Optimisation goal and Billing event

The campaign goal is built considering your advertising goal. It's set at campaign level and will define your optimisation goal and billing event afterwards. If the purpose of the campaign is people installing your mobile app, then you choose Mobile app installs as the goal for your campaign.

The optimisation goal is about setting advertising goals you want to achieve when Facebook delivers your ads. We use your ad set's optimisation goal to decide which people get your ad for the lowest cost. For example with app installs, Facebook delivers your ad to people who are more likely to install your app.

Good to know: Both goals are completely connected. The optimisation goal has a direct link to the campaign goal, so the default options will always be what you want to optimise for.   

Inside the optimisation goal, there is the billing event, which is what you pay for. Impressions is the most common default billing event, and that means that you'll pay when someone sees your ads. If your campaign is a mobile app installs campaign, then you can choose between Impressions and App installs to set as billing event.