IMO it means they're only testing people who are likely to be infected. So they're not testing the general population. For eg, they may only be testing where there are known outbreaks, or only testing people who report to hospital with serious covid-like symptoms.
IMO different testing strategies are perfectly legitimate. It's expensive and slows down limited testing facilities if you try to test the entire population. Can be better to prioritize quick diagnosis precisely where needed.
The R number is not measured, like positivity is, it's calculated based on statistical formulas, plugging in numbers such as the change in hospitalizations or deaths over time.
Coronavirus: What is the R number and how is it calculated?