Apple Push Notification Service vs Courier: What are the differences?
Developers describe Apple Push Notification Service as "A platform notification service *". It is the centerpiece of the remote notifications feature. It is a robust, secure, and highly efficient service for app developers to propagate information to iOS (and, indirectly, watchOS), tvOS, and macOS devices. On the other hand, *Courier** is detailed as "Deliver your notifications to the best channel for each user". Automatically upgrades your outbound user notifications to use the best channel for each user. Email, mobile push, in-app, SMS, or chat — all from a single API.
Apple Push Notification Service and Courier belong to "Web Push Notifications" category of the tech stack.