# Key Points

{% hint style="info" %}
This section describes:

* How Nexus makes use of ISO 20022 messages
* How to use the `acmt.023/acmt.024` messages for proxy and account resolution
* How to use the `pacs.008/pacs.002` messages for payment instructions and acknowledgement
* How instant payment system (IPS) operators that do not use ISO 20022 messages domestically should approach translation to and from ISO 20022
  {% endhint %}

## 60-SECOND SUMMARY

* **Use of ISO 20022:** Nexus exclusively uses ISO 20022 standard messages (and RESTful APIs) to communicate with IPS. The first release of Nexus will use `pacs.008` for payment instructions, `pacs.002` for payment status reports, `acmt.023` for proxy resolution or account resolution requests, and `acmt.024` for proxy resolution and account resolution responses. Further messages are not yet supported but are included in the roadmap.
* **ISO 20022 messages are sent to Nexus via API.** See [ISO 20022 Messages](/apis/iso-20022-messages.md) for further details.
* **CPMI Harmonisation requirements and Nexus message guidelines:** The Nexus usage guidelines are designed to be consistent with the [CPMI Harmonisation Requirements](https://www.bis.org/cpmi/publ/d218.htm) first and foremost, and then consistent with the Cross-Border Payments and Reporting (CBPR+) usage guidelines. Where the CPMI guidelines differ from the CBPR+ guidelines, Nexus follows the CPMI guidelines.
  * Nexus makes some optional elements mandatory in order to enable processing of Nexus payments across two IPS.
* **Compatibility with IP+:** Where the ISO 20022 IP+ message guidelines are followed for domestic instant payments, compatibility with Nexus usage guidelines can be achieved (even before joining Nexus) by ensuring that elements which are mandatory for Nexus payments are optional in the domestic message format (rather than setting a “do not use” rule).
* **Payment tracking:** Every payment instruction sent through Nexus must have a Unique End-to-End Reference (UETR) to allow tracking and investigation of the payment. If Source PSPs are not able to add this UETR, it must be added by the IPS before the message is sent to Nexus.
  * Note: There is no link or shared ID between the proxy resolution or the account verification and the following payment instruction.
* **Use of domestic messages, and message translation:** The Nexus Scheme Rulebook does not require IPSs to use ISO 20022 to communicate domestically with their own PSPs. However, IPSs who do not use ISO 20022 for the domestic processing of Nexus payments must translate those domestic messages to be compatible with the Nexus usage guidelines **before** sending messages to Nexus, and back to the format used in the domestic leg **after** receiving messages from Nexus. Nexus does not provide a translation service; this must be handled by the IPS.
* **Use of ISO 20022 external codes:** Where a code is used to denote a status, reason for rejection/failure, or proxy type, Nexus uses codes from the [ISO 20022 External Code Set](https://www.iso20022.org/catalogue-messages/additional-content-messages/external-code-sets). Proprietary codes are not accepted. If proprietary codes are used domestically, they must be translated to the relevant ISO 20022 external code before the message is sent to Nexus.
* **Message transformation:** “Transformation” is different from translation. Whereas translation moves the data unchanged from one message format to another, transformation in Nexus may change the value or position of some elements of the message.
* Some messages need to be transformed by Nexus as they travel between the Source Country and Destination Country (or vice versa). Transformation varies depending on the message type but may include:
  * Moving the agents in an instruction (eg changing the Instructing and Instructed Agents from the Source PSP and Source SAP to the Destination SAP and Destination PSP respectively)
  * Converting a currency and payment value from the Source Currency to the Destination Currency (according to exchange rate given in the message, which will be verified against the Nexus quote ID provided in the message)


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