There is an aspect of the difference between what travel documents are required to board a commercial carrier headed to Canada, versus what is required at the PoE, which involves well-known standards but which are seldom discussed in this context:
-- At the PoE: the government is limited in what it can impose on Canadians at a PoE. Canadian citizens have a charter right to enter Canada. Canadian PRs have a statutory entitlement to enter Canada. CBSA cannot unduly restrict either of these rights. Thus, once a person is identified as a Canadian, either a citizen or PR, CBSA must allow entry.
-- Regulation of commercial carriers: the scope of powers to regulate commercial carriers is far broader and (in so far as the law has been interpreted and applied so far) does not invoke the respective entry rights of citizens and PRs since the rules do not directly restrict or limit actual entry into Canada.
I do not know if anyone has ever challenged this. It is at least arguable that these restrictions unduly impede a PR's right to enter Canada, by in practice impeding the PR's capacity to reach Canada. But my guess is that the courts would quickly reject this argument based on the governmental interest in regulating commercial traffic coming to Canada.
In any event, bottom-line is that the government exercises broader power to restrict who can board a commercial carrier headed to Canada than it has to restrict either citizens or PRs from entering Canada. Thus, in particular, any evidence (rather than a particular document) which suffices to establish a person's identity and status (as citizen or PR) requires CBSA to allow entry. In contrast, to board a flight headed to Canada, very specific documents, certain "prescribed documents," must be presented in order to be allowed to board the flight.
Going forward, airlines do not do the screening of passengers independently, but rather all passengers' information (API) must be submitted to the IAPI system and a boarding pass will be issued only if the IAPI system (operated by CBSA) provides clearance. Indeed, IRCC is cautioning even Canadian citizens, who are also the citizen of another country, that by fall even they will not be allowed to board a flight to Canada without a proper Canadian passport or Canadian Travel Document, regardless what other passport they carry. (Whether this actually will be enforced against those who are dual U.S. and Canadian citizens is uncertain. It is clear the plan is to enforce it for dual citizens with a visa-exempt passport from another country.)
