In the mobile application industry, it is common to consider retention rates as metric to measure the engagement of users.
The common measurements are D1, D7, D30 for retention rate at 1, 7 and 30 days after the initial app launch.
And we decided to tie our fee only to the users that engaged enough to your game.
So, Returned Device measures the devices who return to your game within a week (7 days) post-initial activation.
It means that users that launch your game only once within 7 days are not counted and charged.