There are three kinds of Kerberos delegation in Active Directory:

  • Unconstrained
    When a Domain Administrator configures a service’s account to be trusted for unconstrained delegation, that service has the ability to impersonate any user account to any other service. This is the most insecure delegation option, because a service could impersonate any user to any other service it likes. For a regular user account, not so bad, but for a Domain Admin or an Enterprise Admin, a rogue service could request information from the domain or change user account or group permissions in the name of the privileged account. For this reason, unconstrained Kerberos delegation is a high security risk.
  • Constrained
    First introduced with Windows Server 2003, constrained delegation allows an administrator to limit the services to which an impersonated account can connect to. Constrained delegation is difficult to configure and requires unique SPN’s to be registered as well as Domain Admin rights to implement. Constrained delegation cannot cross domain or forest boundaries.
  • Resource-based Constrained
    First introduced with Windows Server 2012, Resource-based constrained delegation improved on the constrained delegation introduced with Windows Server 2003. It eliminated the need for SPNs by switching to security descriptors. This removed the need for Domain Admin rights to implement and allowed server administrators of backend services to control which service principals can request Kerberos tickets for another user. Resource based allows delegation across domain and forest boundaries.

For more information on Kerberos delegation, refer to this documentation:

Kerberos Constrained Delegation Overview