How many people on average those who test positive have been in contact with during the relevant period.
And potentially, how many of their contacts go on to catch it.... but only if we test them all. The current plan, I think is to only test those who go on to develop symptoms.
Why do I think this is important? I think it could help inform decision-making, especially for those who are more vulnerable and their families. What is the probability that you will be infected if you spend 15 minutes with someone who is themselves infected?