Tested means a product in question has been tested by Roon in their test lab to work with Roon. A Roon tested product can be a device that connects via a physical connection such as USB or HDMI to a Roon Ready, Roon bridge or Roon core device allowing audio playback usually DACs and AV Receivers. In these cases RAAT is used as the transport between core software and the output via USB or HDMI.
Roon tested also includes devices that Roon can play to over the network that do not use RAAT. These can use AirPlay; Chromecast, Sonos, Squeezebox,Kef Streaming, Linn Songcast.
However not all DACs or AV Receivers, AirPlay, Chromecast or Sonos devices are classed as Roon Tested as there are so many out there that Roon have not had in to test. It relies on the manufacturers to want them tested and send them in, but as they use the same protocols they will show up and more than likely work fine. For network connected devices these show up under Other Network Devices and have no artwork to depict them and Roon doesn’t know there full specs. Dacs or AV Receivers show up only connected to an existing RAAT based transport such as pc/streamer that is Roon Ready, uses Roon Bridge or is connected to the core again Roon doesn’t know much about the device other than what it broadcasts it supports so often some features are not available or it misreports it’s capabilities.