I am a junior in the US applying to both US and Canada schools.
My school grades a bit weirdly - they only give out As, Bs, etc. An A is anything from a 90 to over 100, so there are no A+s and A-s.
I know that Canada schools place a big emphasis on grades, but how would they determine my actual grades if my school just gives out letter grades with no pluses or minuses? Are they allowed to request for my actual grades? How would they evaluate me compared to other people who have pluses and minuses, or Canadian students, who have “averages” like 94, 95, etc?
In general they would just look at your grades on the transcript and evaluate within the context of your school. Your GC may provide more texture regarding your schools grading scheme and how you rank relative to your peers.
“Allowed” is not the right verb to use. But an AO can certainly ask your GC for further information.
A school profile which describes the grading system, levels of classes offered etc. is sent along with each transcript. Admissions officers are used to handling different grading systems.
For what it is worth, one of my daughters went to a high school in the US that only gives out letter grades, and which also does not rank (although we do know where she would have ranked). She is currently in university in Canada (is between her freshman and sophomore years). She got in everywhere she applied, and got some merit aid.
I am not sure how admissions in Canada deals with all letter grades, but they seemed to be fine with it. My understanding agrees with @skieurope and @happy1, that they get a profile from the school so that they will know how your grades compare with others from your school.
However, the main impression that I had was that the application process at Canadian schools was very straightforward. Also, we found the people who work for admissions at various Canadian schools to be very helpful.