What is a user-agent?
One type of data your email tracking pixel and tracking links can collect are user-agents. Here's a definition: "The User-Agent request header contains a characteristic string that allows the network protocol peers to identify the application type, operating system, software vendor or software version of the requesting software user agent" (from: Mozilla). So, it is some piece of data that tracking pixels and URLs retrieve that tells us something about the device and software of the recipient of the pixel.
Based on the user-agents, we determine what kind of device, browser and OS (operating system) your recipients use. We visualize the device type in the "Devices" donut chart. From your raw data and CSV-export you can see the actual user-agents.
Learn more on why tracked devices may not always be accurate.