GTS table linking the commodity code/tariff num to the declaration

Does anyone know the GTS table linking the commodity code / tariff number field (ccnst or ccncu) to the customs declaration item.  I have search through the tables with entries of doman ccngn looking for a GUID reference but cannot find a link to CUIT.

We want to write out the text description in an IDOC IWTX but cannot find the link.

Thanks

Chris