Yahoo and AOL run a Complaint Feedback Loop (CFL) service that forwards recipient spam complaints back to the sender. Registering for that service used to be something senders did themselves. Not any more. Act-On handles the registration on your behalf, and there's nothing for you to configure.
Who does this: Nobody on your side. Act-On's deliverability infrastructure is already registered
Time needed: Zero - no action required
Why this matters: Even though you don't have to register, knowing CFL exists helps you interpret Act-On's complaint reports. When a Yahoo or AOL recipient marks your message as spam, that complaint flows back through the feedback loop, Act-On records it, and it surfaces in your deliverability reporting and suppression lists automatically.
In plain English: When a Yahoo user clicks "This is spam," Yahoo tells Act-On. Act-On automatically suppresses that contact from future sends, so you don't keep mailing someone who already said "stop." You don't have to do anything to turn this on.
Learn more about Yahoo's CFL
If you want background on how the Complaint Feedback Loop works on the provider side, Yahoo publishes the details here: Yahoo Complaint Feedback Loop.