PubNub counts a daily active device (DAD) by tracking the UUID that is used to initialize PubNub in the app (mobile device, web browser, desktop, server, embedded device, etc) that hits the PubNub Network within a a 24-hour period.

For monthly billing calculations, the day with the highest Daily Active Devices count in the given month is used. For a more accurate device count, you should explicitly set the UUID when you init PubNub.