The ENCT (External Neutral Current Transformer) in NSX or the External Neutral Sensor in the ACB's is required depending on your application / requirement.
If you need Neutral protection or Earth Fault Protection / Ground Fault Protection then the Neutral CT is mandatory with 3P NSX or 3P ACB's.

If you have 4 poles breaker then the external neutral is not required.

Please find the attached instruction sheet for more information.