The customs duty regime in Canada is based upon self-assessments at its foundation. An importer, upon importing goods and accounting for them, will self-assess the amount of duty owing based on the tariff classification, the origin and the value of the goods. Following that, if they've overpaid, they can seek a refund. If they've underpaid, of course, they can make an adjustment request to pay the difference.
Following that refund, and in general, just as a broad principle, the CBSA maintains a risk-based compliance approach. This involves reviewing both individual self-assessments upon import and also refund requests to determine if they are accurate. On occasion, additional information comes to light that indicates that there may be an issue with any individual importation or any individual refund. At that point, normally, in general, additional information can be requested from an importer. Based on any additional information that's provided, the CBSA will make a redetermination.
Following that, if the importer disagrees with whatever that redetermination is, they have the legislative right to appeal, initially to the CBSA's recourse directorate, which ensures an impartial review of whatever decision was made.
Following that, there are additional levels of appeal, both to the Canadian International Trade Tribunal and to the Federal Court of Appeal.
The ultimate goal of the CBSA in assessing duties and taxes is to ensure the accurate assessment at the end of the day of whatever duties are owing.