Google launched Google Newsstand in 2013 as a mobile app that brought together news feeds and digital magazines. In 2018, Google merged it with Google News and the News section of Google Search into a single, updated Google News app. Newsstand's features now sit in the Following and Newsstand tabs of that app, where users can subscribe to the feeds of specific publishers. Because it's a Google news service, its traffic is grouped with search traffic.
Piano Analytics automatically classifies traffic from Google Newsstand in the Search engines source, in the Search category. It identifies this traffic from the referrer URL, https://play.google.com/newsstand.
If you want to track Google Newsstand apart from search engines, create a source group. A source group is a custom property that groups and renames traffic sources according to rules you define. For example, a publisher who relies on news aggregators can create an "Aggregators" source that collects visits whose referrer contains play.google.com/newsstand. They can then compare that traffic with their organic search traffic.
Source groups are calculated each time you query them, so they also apply to your past data. To create one, go to Data Management > Sources. For the full setup and limits, see Source groups.