First we need to understand the different alert levels attached to notifications. Here is short description of notification alert levels from https://www.makeuseof.com/ios-15-time-sensitive-notifications-explained/. It refers to iOS, but applies equally to macOS.
Apple categorizes notification interruptions into four levels: passive, active, time-sensitive, and critical. Let's break down each to help you understand them better.
- Passive: iOS delivers passive notifications (those that don't need immediate attention) silently without triggering sounds and
vibrations and lighting up the screen.
- Active, the default interruption level, are the regular notifications that will produce sound and vibrations and light up the
- Time-sensitive notifications are the special kind that will alert you, like active ones, but with a high priority of breaking through
enabled system controls like Focus mode and Notification Summary.
- Critical notifications, first introduced in iOS 14, have a higher priority than time-sensitive as they can bypass the ringer switch
alert. Critical notifications only apply to sensitive cases like
severe weather alerts and local safety alerts. And due to their
nature, Apple has to grant an app developer permission individually
to deliver critical notifications.
Note the reference to Focus mode. Time-sensitive alerts are intended to be those that the developer believes you might want to break through when you have a particular focus mode (e.g. Do Not Disturb) enabled.
You can control the delivery of time sensitive alerts by both application and focus mode. This is in
System Preferences > Notifications & Focus.
It is up to the app developer to decide on the alert level for its own notifications. In typical Apple style, I can't find a precise definition or technical explanation for what Calendar app considers to be Time-sensitive. In Help, Apple say this for Calendar's Time-sensitive alerts:
Notifications for events that require your immediate attention.
And that is the best I can do. Not what you wanted, I know!