Facebook custom audiences from a customer list is a type of audience you can create made up of your existing customers.
Learn more about how to make a Custom Audience in Facebook.
You can leverage your custom audiences from Facebook to build new Strategies in Lightning AI. You can also use Lookalike audiences.
Using custom and lookalike audiences are a great way to get started with Lightning AI because it gives the platform a starting point with a relevant data set.
When you've made it to the "Advanced" section of the Strategy Build, select "custom_audiences" from Advanced Targeting.
Select your desired Custom Audience and be sure to select whether or not this is a Lookalike audience as well.