Transfer / Trade NACK by:

In the case that a transfer / trade needs to be canceled, this can be done by different parties:

REGISTRY: Means that the domain-registry is able to approve the transfer.

REGISTRAR: Means that the losing registrar is able to deny the transfer.

REGISTRANT / CUSTOMER: In some cases, the registrant is able to cancel a transfer / trade.

BOTH: Both parties are able to cancel the transfer.

NONE: Nobody has to approve the transfer.