We're using «News Service on Refinitiv Data Platform - User and Design Guide, v2.0» to implement our news-handling service which fetches news using AWS SQS.
The service subscribes to receive updates of complete news stories (that is, it uses the /news-stories-subscriptions endpoint for subscribing to alerts).
We need to have a way to uniquely identify each alert (that is, the delivery of a news story update) received via the AWS SQS queue created when we have subscribed.
The only piece of the payload JSON data which appears to fit is the "ecpMessageID" field of the top-level object in the payload JSON document.
Unfortunately, it is not documented in the manual we're using (mentioned above), and we have failed to find any mention of it in the NewsML-G2 spec  the JSON payload is supposed to follow. Searching this Q&A site also yielded nothing.
So can you please explain the following?
The "ecpMessageID" is a story ID that you can use to retrieve a story from the static news service. Given you are subscribing to the stories-subscriptions service, this field should always be present. In terms of uniqueness, I believe it should also be unique. I also noticed the "sourceSeqNo" property which may also fall under this category.
That being said, I would suggest you ask this question in Refinitiv Support and they will involve a News specialist who can confirm the meaning behind these fields and suggest the appropriate one to use to represent uniqueness. But my sense is you should be able to use the "ecpMessageID".