The URL of the RER service is registered for the events it handles.
There are two ways to register a handler: Note RERs have the same purpose as event receivers in farm solutions; but event receivers have custom code that runs on the Share Point servers, so they cannot be used in Share Point Add-ins.
So you have to think alternatives before start validation coding in event handlers.
The library was configured with multiple Content Types, but the issue occurred only on some of them.
It appeared that for those Content Types, the Managed Metadata field was Optional, not Required.
Today I was troubleshooting a customer farm where Managed Metadata would remain empty in Share Point, even though it was filled in correctly in the document’s Document Information Panel.
Digging through the internal XML structures of the DOCX and also the Content Type Schema and Field XML, I couldn’t find a reasonable explanation.
Your add-in can handle the following list and document library events.