Do I have to add carriers? Does DTN TABS require carriers?

A carrier record is required only when you want DTN TABS to authenticate carriers.

You can use the Carrier Setup page (Data Management > Carriers) to add and manage carriers. DTN TABS defines a carrier as the transporter of the product. The carrier is also the entity responsible for requesting authorization from the seller on behalf of the consignee. In some cases, the carrier may be the customer.