When a cardiologist writes the term "rare" (for example: rare sinus pauses, or rare sve beats) in a report, do they mean rare as in 'not very common amongst the population' or rare as in 'not occurring in the patient very often'?

This will depend on the Dr. To me rare means I have never seen it in 60 years of practice. I would invest $20 at uptodate.com and read what the worlds leading medical reference has to say about it. What you want to know is how often does this occur in people like you: age, BMI, ethnicity, other illnesses, other meds etc.

B or “not occurring in the patient very often”, such findings, sinus pauses or extrasystoles are otherwise extremely common in the general population.