Teradata recommends that all users are mapped to a defined routing rule using a user mapping. Unity includes a default routing rule that is used when a user is not mapped to a defined routing rule. After Unity is installed, the 'DefaultRouting' rule is set to Reject. This is a best practice to make sure that any user not mapped to a user mapping is blocked.
The Default Routing rule can be updated to choose a different default behavior. For example, you can have users default to a passive, single-system routing rule.
When a user is mapped to the default routing rule, an alert is triggered to inform the DBA. This alert can be disabled, if this is intended behavior. However, this is not a recommended best practice.