If you're using Cyberday via Teams, you can invite people from other email domains to access Cyberday via your Teams environment. This is beneficial e.g. when you work with an auditor in a Teams team in your tenant.
When you do this, the related email domain is added as an allowed domain to your account. You can now see and control these domains, if you e.g. later need to remove an allowed domain.
P.s. Only users you have separately invited and see on the User management -table from the related domains can access the account.