The six objectives and what each one optimises for
The objective is an instruction to the algorithm about which people to find.
Awareness shows the ad to people most likely to remember it - judge it by attention, not sales. Traffic optimises for the click and visit (site, app, Messenger or WhatsApp). Engagement chases comments, shares, messages, video views - middle funnel. Leads collects contact details by instant form, sign-up or message - for services with a follow-up team. App promotion is only for businesses with an app. Sales targets likely buyers and leans on tracking, so it "works best once you have some kind of traffic and tracking history in place."
Picking the objective is picking who Meta will spend your money reaching.