The Leads objective is a new objective-driven Meta objective that offers a new way to create old Lead Generation and a part of old Conversion campaigns.
When to use this objective?
Whenever the goal of your campaign is to generate more leads - either by using a lead form or by generating leads on your website or app.
What's different in comparison to old Meta objectives?
By default, this form sets you up for the old Lead Generation flow and you can create static single and carousel ads with a lead form.
The new thing is the Conversions part on the ad set level. Here, the Lead Form selection keeps this flow, but Web or App selection redirects to the old Conversions flow.
If you enter the Conversions flow (by selecting Web or App), you will be able to create static or dynamic single ad, static or dynamic carousel, or a classic collection ad.
How does this conversion flow (Conversion: Web or App) differ from the conversion flow in Sales objective?
Since the new objectives are objective-driven, the only thing that differs is the purpose of these campaigns. So, when thinking about choosing an objective type, just replace "What?" with "Why?"
For example, in old objectives if you wanted to do some kind of conversion you would make a campaign using Conversions objective.
In ODAX, the question is - do I want to create a campaign to sell or to gather leads?
If you want to sell, you will use the Sales objective and there you will have all the relevant conversion events, like Purchase or Add to Cart.
If you want to gather Leads, then you will use the Leads objective, where you will be offered a different set of conversion events, like Lead or Complete registration.
If you are not sure which objective to use, consult the official Meta documents on conversion events availability per objective.