If your client does not understand what a commodity code is then they use either a different word like HTS code or import/export code or HS code or statistical goods number or custom tariff code, or they had never dealt with this at all since their carrier did all the export/import and customs processing, or you just got not the right person into your team.
You could Google an official page e.g. using the search term "south africa commodity codes" you could get to Export Help - HS Classification Codes - the HS in South Africa
Showing this to your client may enable him to remember.
And if not, then maybe you have to understand that their company does not need this for their business. You could ask them how import / export is done currently with their legacy system.