The Event Action value for downloads stores authentication data. This data includes various authentication methods such as Subscription, InAppSingleIssue, InAppSubscription, and voucher. You can find this data by following this article:
The authentication data type can sometimes show the value 'Unknown'. This value shows whenever the authentication has been disabled. This can happen for various reasons, which often includes errors with the validation. If you find this value in Google Analytics, feel free to contact us to look into the matter.