Mandate/DDI statuses and how they impact Direct Debit Collection are fully outlined here.
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Overview

Mandates/DDIs can have various statuses, which are summarised below:

Status Description
PENDING The mandate/DDI has been created but the payer has not signed. Direct Debits cannot be created against the mandate/DDI.
READY_FOR_EXPORT Bacs-only status; the DDI has been created, the payer has signed but the DDI has not been exported to the AUDDIS service; Nuapay exports DDIs once per day
EXPORTED Bacs-only status; the DDI has been passed to the AUDDIS service
ACTIVE The SEPA mandate has been signed and Direct Debits can be created against it or the DDI has reached Day 5 in the Bacs cycle, so payments may now be made against it.
CANCELLED The mandate/DDI is no longer active. Direct Debit payments cannot be made against a Cancelled mandate/DDI.
UNREADABLE If you are using our 3rd party paper mandate/DDI handling and a mandate/DDI is returned by your payer but cannot be clearly read then it is updated to a status of unreadable.
UNSIGNED This status is applied if you are using our 3rd party paper mandate/DDI handling service and a mandate/DDI is returned by your payer but has not been signed.
SUSPENDED The mandate/DDI is temporarily unusable. Unlike a cancelled mandate/DDI, a suspended mandate/DDI may be re-activated later if required. No Direct Debit payments may be created against a suspended mandate/DDI. Note that you cannot currently suspend a mandate/DDI via the API - this can only be managed via the Nuapay user interface.