Batch status
Sent once per batch created with webhook_mode: BATCH, when every recipient has a send result (usually 1 to 3 minutes after the request). If a recipient is still pending after 30 minutes, the webhook is sent anyway with that recipient as PROCESSING or QUEUED.
Any change after this webhook (a DELIVERED, a READ or a late FAILED reported by WhatsApp) is notified through the regular messageStatus webhook with batch_id; keep the most recent datetime of each message_id. A batch that could not be processed is notified with status: FAILED, an error and the unprocessed list, in any webhook_mode.
Payload
Always BATCH. Distinguishes this webhook from the individual ones.
Number of recipients per status (COMPLETED only).
Reason of the failure (FAILED only).
message_id that were never processed (FAILED only).

