They are used to:

• Complete declarations and paperwork
• Check Duty or VAT requirements
• Check for duty reliefs.

In Sage 200cloud you can record commodity codes against your Stock Items:

• Go to Stock Control, then Stock Records, then Amend Stock Item Detail and open the Analysis Details tab.
  
  NOTE: The length of the commodity code field was increased to up to 10 characters in the Summer 2019 enhancements release.  If you are using an older version of Sage 200 and you require the ability to enter a 10 digit commodity code you would need to either upgrade or speak to a Sage developer to look into extending the field for you, please contact your Sage Business Partner to discuss your optiions.  
  
  If you need to add a  commodity code to a number of Stock Item records it is possible to export your Stock Item add the commodity codes to the file and re-import the Stock Items, for further details see here.