Audience targeting
With audience targeting, you target an audience that you have already built out in your account. The most obvious use of this is your remarketing audience to expand the scope of your remarketing.
Another, sneakier way to use audience targeting is to upload your existing email list and use similar audiences to expand the scope and size of your list. In effect, this is almost an artificial way to expand your email list and reach thousands-to-millions of potential new customers.