This article will explain the difference between the domain contact and 123-reg account contact.
The Domain contact is the legal owner of the domain name as shown on a WHOIS lookup as the registrant, this can be an individual or a company/organisation name as required. If you are managing the domain name on behalf of another party you would use their details as the Domain contact and yours as the Admin contact.
For help with changing your domain name contact details please see the guide:
How do I modify the contact information for my domain name?
The Account contact handles the day to day management of the 123-reg user account. This is where emails relating to renewal notifications, domain expiration reminders and other important emails from 123-reg will be sent
For help with changing your 123-reg Account details please see the guide:
How do I change contact details for my 123-reg account?