A message ID is the unique identifier of one message, used to match delivery receipts, read events and replies back to the send that caused them.
Who assigns it
In RCS Business Messaging the sending agent chooses the ID, a unique string such as a UUID, and passes it with the request. Every later event about that message, delivered, read, expired, or a tapped suggested reply, carries the same ID, so the business's webhook can update the right record without guessing. Most SMS APIs work the other way round and return a provider-generated ID in the response; a platform that handles both channels maps the two.
What it enables
Exact attribution. A delivery rate is a count of IDs with a delivery receipt over IDs submitted; a click-through rate is IDs with a tap event over IDs delivered. Idempotent retries: resending with the same ID after a timeout does not create a duplicate, because the platform recognises it. And support: when a customer asks "did you send me something on Tuesday", the ID is what the log is searched by.
What to store
Keep the message ID, the recipient, the timestamp and the campaign together, and keep the consent record next to them; the compliance checklist explains what an auditor will ask for. The SimplyRCS API returns the ID on every send and includes it in every event.