If you are traveling in transit to another destination but traveling through Canada then you are required to apply for and have a valid eTA approval.
You still need to apply for a travel authorization through the eTA process even if you only have a layover in Canada on your journey to another destination.
For example, if you are traveling from the UK to Mexico via Toronto, a valid eTA is still required.
We have supplied a field in the eTA form to indicate this situation, and you should select “yes” if your travel to the Canada occurs in transit to another country.