# General Usage of ISO 20022

This guide defines the message standards that consists of message elements:

* required in the Nexus Scheme Rulebook as business requirements
* needed for processing by Payment Service Providers (PSPs) and Instant Payment Systems (IPS)

These message elements define the Nexus service and are denoted in the specific ISO 20022 Nexus message guidelines. This section describes the relevant Nexus requirements, such as the use of the message element, its components or the values that must be used. Usage rules, for example, may indicate limits on the number of repetitions, or code value restrictions, while format rules may be used to indicate the allowable combinations of components of a message element.

## Only ISO 20022 messages are accepted <a href="#toc159257042" id="toc159257042"></a>

Nexus expects and will only accept the Nexus-standard ISO 20022 messages (and JSON APIs as defined by Nexus). These messages are based on the [CPMI ISO 20022 harmonisation requirements](https://www.bis.org/cpmi/publ/d218.htm) and CBPR+ usage guidelines. The Nexus Usage Guidelines adds additional requirements necessary for processing Nexus payments.

{% hint style="info" %}
Where an IPS community either (a) does not support ISO 20022 messages, or (b) uses elements differently than defined in the Nexus usage guidelines, then the IPSO is responsible for translating messages to be consistent with the Nexus usage guidelines. See [Translation To/From Domestic Message Formats](/messaging-and-translation/translation-to-from-domestic-message-formats.md)for further detail.
{% endhint %}

## Accepted Message Set <a href="#toc159257043" id="toc159257043"></a>

With the exception of some JSON API calls, most communication through Nexus is done using messages following the ISO 20022 standard. The following ISO 20022 messages are implemented in the first release of Nexus:

* [**`pacs.008.001.13`**](/messaging-and-translation/message-pacs.008-fi-to-fi-customer-credit-transfer.md) **FI to FI Customer Credit Transfer**
  * This is the basic payment instruction
* [**`pacs.002.001.15`**](/messaging-and-translation/message-pacs.002-payment-status-report.md) **FI to FI Payment Status Report**
  * This is the payment status report returned in response to a `pacs.008`
* [**`acmt.023.001.04`**](/messaging-and-translation/message-acmt.023-identification-verification-request.md) **Identification Verification Request**
  * Used for proxy resolution and account resolution requests – see [Addressing & Proxy Resolution](/addressing-and-proxy-resolution/key-points.md).
* [**`acmt.024.001.04`**](/messaging-and-translation/message-acmt.024-identification-verification-report.md) **Identification Verification Response**
  * Used for responses to proxy resolution and account resolution requests.

The following messages are **not** implemented in the first release of Nexus, but are on the roadmap for future releases:

* **`pacs.028` FI to FI Payment Status Request**
  * Request for an update on the status of a previously sent `pacs.008` payment instruction
* **`camt.056` FI to FI Payment Cancellation Request**
  * In Nexus, this would be used to request the return of a previously completed payment. (It is not possible to request the cancellation of a payment that is currently being processed.)
* **`camt.029` Resolution of Investigation**
  * This is the positive or negative response sent in reply to a `camt.056` Payment Cancellation Request
* [**`pacs.004`**](/messaging-and-translation/message-pacs.004-payment-return-not-yet-supported.md) **Payment Return**
  * This is a payment instruction used to return an earlier payment. It is very similar to the `pacs.008` message structure. A `pacs.004` message references the original `pacs.008` so that the PSP receiving the returned payment can reconcile it against the original payment.

## Version <a href="#toc159257044" id="toc159257044"></a>

Nexus uses the 2025 version of the ISO 20022 XML message standards. This is version 13 for the `pacs.008` and version 15 for the `pacs.002`.

### Backwards compatibility <a href="#toc159257045" id="toc159257045"></a>

Nexus will not support backward compatibility. Any translation between previous versions and the current version will be the responsibility of the IPSO. Nexus will define specific cut-over moments when new versions are introduced.

## Character Set <a href="#toc159257050" id="toc159257050"></a>

The efficient processing of cross-border payments depends on the use of a common character set so that all participants in the processing chain will be able to understand and interpret the information. Otherwise, payments risk being delayed or even returned.

Following CPMI harmonization requirements, Nexus prescribes the use of a restricted character set in its cross-border payment messages to the currently agreed Latin character set: lower case characters a–z, upper case characters A–Z, numeric characters 0–9, complemented with the following additional characters for a limited selection of data elements:

/ - ? : ( ) . , ’ + ! # & % \* = ^ \_ \` { | } \~ " ; @ \[ \ ] $ > <

References, identifications and identifiers must respect the following:

* Content is restricted to the Latin character set as defined above
* Content must not start or end with a forward slash ‘/’
* Content must not contain a double forward slash ‘//’

## Message Validation <a href="#toc159257051" id="toc159257051"></a>

Messages sent to Nexus will be validated against the requirements in the Nexus message usage guidelines. Messages will fail validation in the following cases:

* The message does not conform to the Nexus XSD validation. In this case, Nexus will reject the message with error code FF01.
* An element that is defined as “Mandatory” in the Nexus usage guidelines is empty or null, or the entire element is missing from the message structure. In this case, Nexus will reject the message with error code `CH21`.
* An element that requires a code from the ISO 20022 External Code set in the \<Cd> element instead uses the \<Prtry> (Proprietary) element. In this case, Nexus will reject the message with error code `CH21`.
* An element that requires a code from the ISO 20022 External Code set uses a code that is not in the External Code Set. In this case, Nexus will reject the message with the applicable error code.


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