Custom domain means that you and your customers don't use example.effect.io as domain, but your own selected domain like
www.example.com.If your subscription allows it, we can help you set up this.

All subdomains like app.example.com,
www.example.com etc are allowed. But not root domains like example.com.
From Support, you will receive two rows of records you need to set up at your DNS provider. If you don't know what that is, contact your IT-department.
One row is the actual redirect and second is validation of SSL (required).