Several exemptions from the requirement for pre-approval to travel to Canada are in place, including:

U.S. Nationals: Citizens of the United States.
Royal Family: Her Majesty the Queen of Canada and any member of the Royal Family.
Residents of St. Pierre and Miquelon: Citizens of France residing in St. Pierre and Miquelon who wish to enter Canada directly from there.
Re-entering Visitors: Visitors, students, and workers returning to Canada after solely visiting the United States or St. Pierre and Miquelon, as long as they return before the end of their authorized stay or any extension.
Refueling Flights: Foreign nationals on a flight stopping in Canada solely to refuel, provided they have the necessary documents to enter the U.S., or if their flight originated from the U.S. after lawful admission.
Emergency Landings: Foreign nationals on flights making unscheduled stops in Canada due to emergencies or unforeseen circumstances.
Transit Programs: Foreign nationals transiting through Canada under the Transit Without Visa or China Transit Program.
Diplomatic Passports: Holders of passports with diplomatic, consular, or official acceptance issued by the Chief of Protocol for the Government of Canada, who are accredited diplomats or officials of a foreign country, the UN, or any of its agencies.
Transportation Crew Members: Foreign nationals entering Canada as crew members of a transportation vehicle, either to join a crew or to transit after working, provided they have a ticket for departure within 24 hours of arrival.
Military Duties: Foreign nationals entering Canada solely to fulfill official duties as armed forces members from designated states under the Visiting Forces Act, unless designated as part of a civilian component.
Aviation Inspectors: Foreign nationals entering Canada solely to inspect flight operation procedures or cabin safety of international commercial air carriers, if they are civil aviation inspectors with valid documentation.
Accident Investigations: Foreign nationals participating as accredited representatives or advisers in aviation accident investigations under the Canadian Transportation Accident Investigation and Safety Board Act, provided they have the necessary documentation.