This is actually a HUGE subject with many tangents.
For the vast majority of grant citizenship applicants, however, it is simple. And it is the underlying reasoning hard-wired into the presence calculation. ASSUMING the applicant accurately reports ALL dates of exit and dates of entry into Canada, the online presence calculator counts all days as present IN Canada to include all days between a known date of entry and the next reported date of exit, including the date of entry and the date of exit. That's it.
And, indeed, if the applicant does in fact accurately report each and every date of entry and date of exit, there is no doubt, the INFERENCE of actual presence in Canada between those dates is valid and the calculation of total days present is accurate. Actual physical presence shown.
But of course IRCC will examine and consider and compare a lot of other information to assess the completeness and accuracy of this information. This is what the applicant's address and work history is largely about. Is that information consistent with the applicant's reported travel history? If a processing agent or Citizenship Officer notices some inconsistency or incongruity regarding this information, that tends to trigger a more in-depth examination of what the applicant reports AND potentially triggering more in-depth examination of other information, including potentially what is called "open source" information. The latter can include inquiries researching LinkedIn, for example, or social media platforms. I reference "LinkedIn" because it is one of the more common sources of information which CIC/IRCC has accessed triggering negative decisions.
For the vast majority of applicants it tends to be easy to see that all the applicant's information (including immigration history, address history, work history, and other information) is consistent with the applicant's reported travel history. And thus, if otherwise IRCC does not see any indication the travel history is incomplete or inaccurate, it is easy to conclude the travel history is accurate and complete . . . leading to the inference that the applicant was IN Canada all days in-between a reported date of entry and the next reported date of exit.
BUT IF AND WHEN there is some indication the applicant MAY (just possibly) not have accurately reported ALL dates of travel, there are, then, reasons-to-question-presence/residency. As I noted in the other post, specific criteria constituting reasons-to-question-presence/residency were publicly disclosed until 2012, and then the criteria adopted, the "triage criteria," became known through a leaked copy of the FRC (File Requirements Checklist) from OB 407. We know the latter has been significantly modified since 2012 but it has been years since we have seen reliable versions of internal IRCC documentation regarding the triage criteria, so the precise criteria now in use remains largely secret.
Some reasons-to-question-residency are obvious. If and when IRCC discovers the applicant omitted a trip abroad, duh, obviously that triggers elevated scrutiny and can lead to RQ-related non-routine processing. The typical way IRCC discovers this is by comparing the applicant's reported travel history with the CBSA record of entry into Canada. But of course that is NOT the only way in which IRCC might discover the POSSIBILITY the applicant failed to disclose, or inaccurately reported, a trip abroad.
In the latter regard, it warrants noting and emphasizing that not all errors are created equal or treated equally. There are credible anecdotal reports in this forum of RQ-related non-routing processing being triggered by the failure to report a SINGLE DAY-TRIP to the U.S. In contrast, there are also credible anecdotal reports from applicants who overlooked, and thus left out, trips for many days, even three weeks, without that triggering any non-routine processing at all.
Big factors which undoubtedly loom large in such different outcomes include how much margin over the minimum the applicant applied with, and the IRCC official's general impression of the applicant's credibility. Credibility looms large. Very large.
Which leads to a side-step, back to the matter of updating information such as changes in home address: even if a failure to advise IRCC of a home address change timely (and, of course, truthfully) does not, itself, cause an overt problem, make no mistake about the potential negative influence if the IRCC official evaluating the information suspects, merely SUSPECTS, the applicant is being at all deceptive. This is NOT about what will for-sure happen. This is about RISKS. About the RISK that playing games with information, like one's "home address," can have a negative credibility influence.
Second to technically meeting the specific requirements themselves, the applicant's credibility looms as the next most significant factor determining how things go. In this regard, there is no specific credibility requirement other than what constitutes outright misrepresentation. Moreover, credibility is mostly about appearance, about impression.
BACK TO HOW IRCC CHECKS PRESENCE:
Overall, IRCC's approach is to look for discrepancies, inconsistencies, incongruities, any information that MIGHT indicate the applicant's information is not complete or entirely accurate. How this is done varies. Sometimes, for example there are telephone calls to the applicant (this happened to me) or the applicant's current or past employer. But again, this is mostly an effort to identify whether or not there is some reason to doubt the applicant's information.
Once there is reason to question the applicant's information, and that rises to the level triggering RQ-related non-routine processing, the burden is put on the applicant . . . and when this happens, not only is the applicant required to document all travel history, but depending on the scope of IRCC's doubts the applicant may be required to provide evidence of ACTUAL PRESENCE in all the days between travel history dates. The applicant might NO LONGER benefit from the inference the applicant was IN Canada from a known date of entry until the next reported or known date of exit. At this point it is NOT about how IRCC checks the applicant's presence but what amount of evidence is needed for the applicant to PROVE he or she was ACTUALLY IN Canada all those days in-between travel dates.
SOMETHING TO THINK ABOUT:
Applicant shows up at the test and interview, and one way or another the interviewer determines the applicant has been living abroad (staying abroad longer than merely traveling abroad). Even assuming this happens in a way that the interviewer has no reason to doubt the applicant's credibility. On the application form (or as updated) it says the applicant's home address is xxxx, Canada. But the applicant is now living at zzzzz, France (or the U.S. or wherever). At the very least this reveals the applicant will report a residential address that is different from where the applicant actually is living. What about the address history in the application? Was that merely some address (friend's or family or . . . ) the applicant could "use" as a home address even though the applicant was actually living somewhere else? Is this a question calling for further examination? Often not. But as soon as the applicant shows IRCC he or she is willing to report a "home address" that is not where the applicant actually lives, obviously this MIGHT invite questions about the applicant's other reporting of residential addresses.
THERE ARE GOOD, SOLID REASONS WHY PRUDENT OBSERVERS EMPHASIZE, AGAIN and AGAIN, BE TRUTHFUL.
Those who play games with the information they provide do so at their peril.