A banking and insurance company has "memberid" as the unique user id for customers who...

According to the Adobe documentation, a custom namespace is a namespace that is created by the user to define a specific type or source of identity value that is not available in the predefined namespaces. A custom namespace for memberid would be recommended to fetch profiles for known customers who land on their website from Google organic search and authenticate, because memberid is a unique user id that is specific to this use case and not covered by any predefined namespace. References:https://experienceleague.adobe.com/docs/experience-platform/profile/identity-service/namespace.html?lang=en#custom-namespaces