The official help for this question indicates that it is asking if you have ever applied for a visa to Canada.
Select yes if, in the past, you submitted an application to come to Canada, such as a study permit, work permit or visitor visa.
If you answer Yes to this question, you'll be asked for your Unique Client Identifier, visa or permit number.
If, in the past, you submitted an application to come to Canada such as a study permit or work permit, you will have a Unique Client Identifier (UCI).
Look for this number on letters you received from Citizenship and Immigration Canada or on the immigration document that was issued. Example: study permit, work permit, or visitor visa.
If you do not know what your UCI is, enter your immigration document number. Example: study permit, work permit, or visitor visa number.
You will not have such a number if you have entered Canada under a visa-exemption, further indicating that this is meant for people who applied for visas.