What is the Managed Accounts
What is managed account?
All Atlassian accounts with email addresses from verified domains become managed accounts.
What can they do?
they can get all benefits of Atlassian Access,for example they can do SSO & 2-factor Authentication.
Ref
https://support.atlassian.com/user-management/docs/what-are-managed-accounts/
https://support.atlassian.com/user-management/docs/verify-a-domain-to-manage-accounts/