I apologized upfront in the first post, for how clumsy my attempt would be; that was not rhetorical. There are a lot of loose threads and tangled tangents in this subject, and a lot of imprecise terms. And as I acknowledged in the later post, while I see a need for IRCC to be more clear, I cannot sort out how to do that.. . . but think you've gone down some verbal/logical rabbitholes that actually don't clarify . . .
IRCC does make it clear that Canadian PRs should have a valid PR card when traveling outside Canada, and if for some reason they don't, they will need a PR TD to board a flight back to Canada. Those that follow this oft repeated guidance (which is often stated more definitively, not just that a PR should have a valid PR card, but stating a PR card is needed) will have no document/status problem boarding a flight to Canada.. . . but for people planning their travel: we NEED to know what is expected, most of us, before we book travel, so that we can obtain the necessary documents.
The fact that some PRs have access to means to travel back to Canada without a PR card or PR TD is not something IRCC is broadcasting. Or, it seems, encouraging. Kind of like how CRA does not clearly provide information to taxpayers about loopholes that can be used to avoid owing a tax which, but for overtly taking advantage of the loophole, the taxpayer would ordinarily owe. Those who intend to take advantage in these ways will, generally, need to navigate this on their own . . . neither IRCC nor CRA inclined to help much.
Leading to . . .
My emphasis on there being no rule specifically requiring PRs to present either a valid PR card or PR TD to get a boarding pass may be misleading; hell, it probably is misleading. It is accurate, but as a practical matter, for the vast majority of PRs, after all, the regulatory scheme more or less mandates they have either valid a PR card or PR TD to board a flight to Canada . . . that is, even though there is no rule specifically prescribing it, the vast majority of PRs will need to present either a PR card or PR TD to be allowed boarding a flight to Canada.Okay, there is no rule requiring them to have the PR card or PRTD. But this phrasing leaves out the point that they CANNOT GET the other 'listed documents' because the government of Canada WILL NOT ISSUE them these other listed documents.
I was mostly trying to illuminate the essential mechanics of screening immigration status, and that this depends on having and presenting an acceptable document showing immigration status with authorization to enter Canada. Certain documents get a traveler on the plane, for purposes of immigration status screening. (Recognizing that actual boarding is also subject to other potential reasons for not allowing a particular passenger to board a flight.)
That is, my focus was on the point that a traveler with one of the prescribed documents showing authorization to enter Canada sufficient for the purpose of boarding a flight to Canada will not be denied boarding, that is, not for failing to show such immigration status. (There are other reasons why travelers might be denied boarding a flight to Canada.)
But I should have more clearly acknowledged that it seems quite likely the scheme for screening passengers for authorization to enter Canada prior to boarding, as it applies to PRs in particular, is intended (among other objectives) to facilitate PR RO enforcement. In particular, the impact of the current scheme, as it applies to PRs, is quite likely intended to minimize, even if it does not entirely preclude, PRs traveling to Canada without either a PR card or PR TD. With the further objective of compelling PRs who do not have a currently valid PR card to go through the admissibility screening of a Residency Determination attendant an application for a PR TD. This aspect of the regulatory scheme should not be understated or underestimated.
That said, nonetheless, because there is no rule otherwise, those PRs who can travel to the U.S. can return to Canada via that route without either a PR card or PR TD. And U.S. citizens, in addition to being able to return to Canada by that route, can fly directly to Canada using their U.S. passports.
HOWEVER, the CBSA/IAPI system "board" message is, in effect, merely giving the airline permission to allow boarding (passenger passing not just authorization to enter Canada screening but some background and security screening as well), confirming the passenger's travel document is acceptable for purposes of pre-boarding immigration status screening.
This does not guarantee the passenger clears additional screening criteria that air carriers are responsible for. For example, inconsistencies or other problems in identification and travel documents, perceived as well as objectively, can indeed lead to an airline denying boarding. Moreover, the CBSA/IAPI system most likely can and sometimes does flag travelers, and may issue a "no-board" message for reasons other than the traveler has not presented an accepted document for showing authorization to enter Canada. The scope of information that is included and subject to assessment, in the automated screening process, is remarkably broad. Moreover, the screening is predicated on precise information in the passenger's travel document (typically a passport), so issuing the boarding pass is directly tied to a particular passport (or other type of travel document if applicable).
Thus, for example, while I am not personally acquainted with situations that involve booking an international flight based on one travel document, but then trying to board the flight with a different travel document, it does not surprise me that this can cause problems.
I previously referenced and linked the Guidelines for commercial air carriers for the processing of prescribed traveller information, which is here:
https://www.cbsa-asfc.gc.ca/publications/dm-md/d2/d2-5-11-eng.html which includes a detailed enumeration of the specific information that must be included for passengers boarding inbound flights to Canada. The required information is also prescribed by regulation in Section 5 in the Passenger Information Regulations, as adopted pursuant to the Customs Act, illustrating the inter-connectedness of the many laws that have a role in these procedures. This regulation is here: https://laws-lois.justice.gc.ca/eng/regulations/SOR-2003-219/page-1.html The collected information is used for screening for multiple purposes, not just the traveler's immigration status, but for security, criminality, and customs purposes as well.