这是文档说明的内容：

The priority of the notification. Specify one of the following values:

10–Send the push message immediately. Notifications with this priority
must trigger an alert, sound, or badge on the target device. It is an
error to use this priority for a push notification that contains only
the content-available key.

5—Send the push message at a time that takes into account power
considerations for the device. Notifications with this priority might
be grouped and delivered in bursts. They are throttled, and in some
cases are not delivered.

关于VoIP：

Instead of persistent connections, developers should use the PushKit
framework—APIs that allows an app to receive pushes (notifications
when data is available) from a remote server. Whenever a push is
received, the app is called to action. For example, a VoIP app could
display an alert when a call is received, and provide an option to
accept or reject the call. It could even begin taking precursory steps
to initiate the call, in the event the user decides to accept.

我是否正确理解apns-priority定义了APNS侧的消息队列，而VoIP推送类型定义了设备上发生的事情？

他们的意思是说

It is an error to use this priority for a push notification that contains only the content-available key.

APNS会返回错误还是只是错误的做法？

附言 我使用AWS SNS发送通知，它不支持apns-priority的自定义，但是我很好奇它如何影响流程。

主要区别在于，即使您的应用程序已关闭，voip通知也可以将您的应用程序唤醒为处于终止/挂起状态，而用户通知仅显示警报而无需唤醒您的应用程序以获取更多参考。
Apple Pushkit文档

主要区别是VoIP notification将在未运行或被应用程序切换器终止时启动您的应用，并且您需要处理代码中的通知，而normal notification将立即显示为本地通知，并且不会启动您的应用 应用程式。