Fields #7 (Mode of transport) and #14 (Tariff treatment), which require special number codes.
Fields #12 (Country of origin) and #13 (Place of export) require either a three-letter alphabetic state code on imports from the United States, or a two-letter alphabetic country code on imports from any other country.
Field #17 (Currency code) requires a special alphabetic country respective to the currency being used in the transaction.
There are other exceptions — for example, whether or not your shipment totals less than CAN$2,500, whether it’s arriving specifically from the United States, or the method of transport involved — that may trigger some fields to be filled out or left alone, so be sure to take note of any relevant details, and don’t be afraid to hire a customs broker to help with any uncertainties.