Every customs declaration classifies the goods with an HS code. The code drives duty, quota, licensing and statistical treatment, and it is the field on which every trade dataset in the world is aggregated.
Two properties make it central to evasion. Classification is a judgement, so a plausible but wrong code is deniable in a way that a false quantity is not. And because the trade statistics of both countries are built on these codes, a misclassification that persists is visible as a discrepancy between what one country reports exporting and what the other reports importing.