ReachApp works with ’random duplication’. The assumption is that people who are reached by a medium are not different from people who are not reached.
This means that any second medium in a plan reaches just as many people that are reached as not-reached by the first medium.
Suppose you have a plan where medium 1 has a net reach of 40%. And then there is medium 2 with a net reach of 25%.
ReachApp assumes that medium 2 reaches 25% of the 40% already reached by medium 1. And it also assumes that it reaches 25% of the 60% not yet reached. The last number is 15% and ReachApp adds this percentage to the 40% already reached. Total Net Reach in this example is then 55%.
With Overlap it works similarly: The second medium reaches 25% of the reached as well as the non-reached group of medium 1. So if you take 25% of the 40% already reached, you get 10%. And this is the Overlap.