The same topic exposes the Record Changed event and Record indexed event
The Azure implementation exposes multiple different system events through the same topic.
There are different use cases where this is undesirable.
For instance the notification service should only expose the Record changed events, but it receives all messages and forwards them on. This incurs extra running costs and complexity to clients who need to filter out messages.
The same applies to internal consumers of the event who subscribe directly to the topics.
One solution is to split the different messages between different topics and let clients explicitly chose which to subscribe to. However this close to R3 this is perhaps too big a change.
Another solution then is to add an extra property onto the messages and make use of the filtering mechanisms in service bus/event grid to filter based on this property e.g. message: recordchanged | recordindexed.