The Trusted Domain is a security feature used by 1Stop to specify which domains your application is allowed to make API calls to. You will see it highlighted under Setup > Users and Control, per the below screen. This feature is only used in rare occasions, most often if you've worked with 1Stop to integrate with a third-party service. Please contact 1Stop if you have any questions about Trusted Domains. You do not need to access this feature in any way, unless you are directed to by 1Stop.