The ‘Unique devices’ metric is a Cross-device analytics metric that counts the number of unique unidentified devices and unique virtual devices. Unidentified devices are devices that generated anonymous hits. Unique virtual devices are distinct people identified per device.
For each device, sum all distinct people tied to it (including anonymous if the device contains non-stitched hits).
Note that this metric is not equal to Unique Visitors in non-CDA report suites. For example, a device is shared by 3 different accounts. If all 3 accounts visit your site in a reporting window, the resulting report would show 3 Unique Devices in CDA. The same data outside of CDA would show 1 Unique Visitor.
Viewing this data in a CDA virtual report suite before Replay potentially stitches unauthenticated hits would show: