A test device concurrency determines the number of devices you can use simultaneously. For example, with 5 concurrent devices you can test 5 different device configurations in the same amount of time it would normally take to test 1.
We measure usage in device hours, which is a calculation of all activities on a device initiated by your test. For example, a 12–minute test suite run on 5 device configurations will consume 60 minutes, or 1 device hour. Time spent downloading apps, processing screenshots, and generating test reports does not count as billable time.