"Advertising Identifiers: A user-resettable identification number for advertising associated with a device (e.g., iOS uses the Identifier for Advertising (IDFA) and Android uses Google Advertising ID). ... Engagement Data: Information about (i) your visits to, and interaction and engagement with the services, content and ads on third party websites, applications, platforms and other media channels ("Channels"), and (ii) your interaction with emails including the content and ads therein."
Humble Bundle collects device advertising IDs (IDFA, Google Ad ID) and tracks user engagement across third-party websites, apps, and media channels, enabling cross-service profiling and tracking.