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Field 21 on an MT103 is the Related Reference — used to point at another SWIFT message that this payment relates to. Optional in plain MT103, mandatory in some MT103 STP variants. Draft an MT103 now →
Format: up to 16 characters, same character restrictions as field 20 (letters, digits and slashes; no leading or trailing slash; no double slashes). When present, field 21 typically echoes the field 20 (Sender's Reference) of the earlier SWIFT message that this MT103 is related to — for example, the original payment being amended, the previous MT103 that is being returned, or the inquiry message that prompted this response.
In standard MT103, field 21 is optional. It becomes effectively mandatory in MT103 STP (Straight-Through Processing) when used as a return or amendment message — the receiving bank needs the related reference to match the new message to the original transaction in its system. In MT202 (bank-to-bank cover payment), field 21 is mandatory and must contain the field 20 of the originating MT103 customer payment, creating a formal linkage between the cover payment and the customer payment it settles.
Field 21 is also used in MT192 (Request for Cancellation) and MT195 (Queries) messages, where it references the field 20 of the payment being cancelled or queried. This cross-message referencing creates an auditable chain of messages that the bank's investigations team can follow when tracing a payment problem. Without field 21, a bank receiving an MT192 cannot easily identify which specific payment the cancellation request targets.
In some treasury management systems, field 21 is automatically populated when generating a return or amendment message, pulling the original payment's TRN from the system and inserting it into field 21 of the new message. This automation reduces the risk of human error in cross-referencing messages. However, in simpler banking platforms, field 21 may need to be manually entered by the operator.
From a practical standpoint, you are unlikely to encounter or need to populate field 21 unless you are a bank operations team or corporate treasurer dealing with payment returns, amendments, or investigations. Most customer-initiated MT103 payments leave field 21 empty. If you are dealing with a payment that was returned and want to understand the return chain, the field 21 values in the return messages create a trail back to the original payment.
| Example value | :21:RELATED-REF-PRIOR |
|---|---|
| Valid characters / format | 16x alphanumeric |
| Required on MT103 | Optional |
| Required on MT202 | Mandatory |
| Required on pacs.008 | Optional |
| Notes | Related reference (links cover MT202 to its underlying MT103). |
In returns, amendments, cancellation requests (MT192), status inquiries (MT195), and in MT202 cover payments where it must echo the MT103 field 20. For a standard one-way customer payment that is not referencing any prior message, field 21 is left empty.
No — field 21 is optional on a standard MT103. It becomes mandatory in MT202 and in certain STP-variant messages where a related reference is contractually required by the bilateral agreement between the sending and receiving banks.
When required, field 21 should contain the field 20 (Sender's Reference / TRN) of the earlier SWIFT message this new message relates to. For example, if you are sending a return of payment with TRN "ABC123456789", field 21 of the return message should contain "ABC123456789".
In some messaging contexts (particularly ISO 20022 translations where a related reference is structurally required but not available), the literal value "NOTPROVIDED" is used as a placeholder. However, this should be avoided in MT103 where field 21 is genuinely optional — simply omit it rather than using a placeholder.
If a payment is returned, amended, or cancelled, the field 21 chain creates a complete audit trail: the original MT103 has a field 20, the MT192 cancellation request quotes that value in field 21, and the cancellation response also references it. This chain allows investigations teams at multiple banks to link all the messages about the same payment.
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