A carrier error code is the reason attached by the operator to a failed SMS, such as unknown subscriber, absent subscriber, barred or blocked by filter, mapped by gateways to plain-language failure reasons.
Operators return failures as numeric or short codes from their signalling systems. They differ by operator and protocol, so gateways translate them into a dozen stable reasons: unknown number, absent subscriber, barred, filtered, unsupported, network error. The raw code stays available for the cases where a support team needs it.
The reasons decide the action: remove unknown numbers, retry absent ones later, change sender or content for filtered ones, escalate network errors if they cluster on one operator.
At SMSMeteor: every failed message carries both the raw carrier_code and a failure_reason, in the report, the API and the webhook.