Basically you are looking at the visit number of the visitors: if a visitor has at least one visit with a visit number higher than 1 (2 or higher) the visitor is segmented as "returning visitor". all others are segmented as new visitors. be aware that this is not depending on the timeframe you are looking at rather segmenting the visitors in their whole history!
But: in your question you mention eVar39. I assume that this value might come from your tracking implementation. that means, someone might have installed the plugin getNewRepeat. basically this plugin tracks information and allows the mentioned reporting. Best have a look at your tracking implementation.