hi,
as the subject says: private peering show magenta colored and doesnt show any coverage in ttnmapper. can someone elaborate?
These are typically networks where the gateways are not registered in TTN but instead are registered under a private TTS instance. Such an instance may or may not fully peer with the community network (allowing 2 way traffic) via PacketBroker. The good guys share with the community, less good will receive data captured via community gateways but then not allow the reverse process - where their gateways pass community traffic to PacketBroker and then on to TTN/TTS(Sandbox) for our benefit.
An example of a good peering arrangement is SmartBerks - covering Reading Council and the (mainly) east side of Berkshire with many Gateways; they often capture and pass on data missed by community gateways and with wide deployment significantly enhance the TTN coverage in the region, by the same arrangement if they dont have coverage or if one of their GW’s goes down for a period the TTN GW’s in the area (many mine!) help backfill and provide redundant coverage. This works well in valley areas such as the Thames Valley near me where often the best way to provide to one side of the Valley is actually deploy gateways with a view on the otherside. Here the county boundary (and limit to SmartBerks physical depoyment) is defined by the course of the River Thames. TTN Community gateways both sides of the river can therefore provide infill coverage 
An example of a poor deployment is one, cough Connexin cough, where in one area I noted >>100 GW’s deployed and peering on PacketBroker but only allow ingress to their network - capturing from TTN Community GW’s but not allowing egress to enhance Commnity coverage. Just decimating their network and allowing 1 in 10 to fully peer would have boosted TTN Community coverage massively at little cost to them - not undermining “competative” advantage, but would massively enhance their ESG position and gain them good PR…ah Well! 
It used to be that users simply signed up to TTN Community deployments - still an option from the main site log in page:
But sadly users often just click the “get started free” instance not realising this is a private discovery instance though still potentially peering with TTN(Sandbox)/Community network.
Knowing a private instance is around helps the Communities plan deployments - if there is ‘good’ peering, or if restricted one might contact the GW owner (if you can track them down!) and agree sharing or potentially local area colaboration. When you look on The Things Network Main map you should see all gateways if you zoom into an area. TTN Community page maps do not show peering GW’s only those directly registered so often worth checking all three resources. TTNMapper will show community and private/peering GW’s but the only way to know if there is through traffic and community coverage is to try a device and look at the message Data to see if routed through PacketBroker or do what I do and get a tracker/locator node and get mapping! 