If the Catalyst Modbus port is bridged to a Modbus Plus network is it necessary to enter MB+ routing fields?
No. In fact, the Catalyst does not support MB+ routing fields.
However, bridging interface devices such as the Modicon BridgeMux (BM85) support node mapping tables that include entries for MB+ routing fields. Using a BM85 and specifying routing, it is possible to route across multiple Modbus Plus network segments in order to reach the desired destination address.