@haji6 Every module that supports this SHOULD have documentation that shows how they talk to each other.
Imagine a room full of people. In that room - only one person can speak at a time.
Person A (Calendar) will yell out “I have an Event at 8:30 called Appointment” and write it on it’s part of the whiteboard (Might be nowhere). Then Person B will hear it and write it on a whiteboard where it’s told to for all to see.
This is a simplified version of how the notification system works.
My module (MMM-Multimonth) and others uses the above concept to display data. Why rewrite work when you can use what’s provided?