# BT-21 Invoice note subject code

Code that classifies the subject of an invoice note.

## Quick answer

> **Invoice basics**: Core document identifiers, dates, process identifiers, and high-level invoice context.

- [Validate XML](/en/xrechnung-validator?source=resource&bt=BT-21)
- [Convert invoice](/en/pdf-to-xrechnung?source=resource&bt=BT-21)

## In plain language

Use this only when the invoice note should be machine-classified instead of being plain free text.

## What you usually enter here

Approved note subject code

Typical valid values include: AAI

Optional according to EN 16931-1. If provided, use the approved code list.

## Where the field sits

BT-21 belongs to the Header part of the invoice. UBL 2.1 Syntax: This syntax has no separate element for the field. CII (UN/CEFACT) Syntax: rsm:CrossIndustryInvoice › rsm:ExchangedDocument › ram:IncludedNote › ram:SubjectCode.

## What goes wrong often

Avoid values like: note

## Valid Examples

- AAI

## Invalid Examples

- note

## Related Fields

- [BT-22 Invoice Note](/resources/xrechnung/bt-22-invoice-note)

## XML mapping and field facts

Open the technical reference if you need XML paths, validator rule codes, legal wording, or schema-specific mapping details.

### Field ID:
BT-21

### Data Type:
code

### Cardinality:
0..1

### Requirement:
Optional

### Max Length:
10 characters

## XML Implementation

### UBL 2.1 Syntax
```
Not used in Peppol BIS Billing 3.0 UBL Invoice (BT-21 cardinality is reduced to 0..0).
```

### CII (UN/CEFACT) Syntax
```
/rsm:CrossIndustryInvoice/rsm:ExchangedDocument/ram:IncludedNote/ram:SubjectCode
```

## Create an XRechnung from your PDF

The converter reads the invoice and shows the fields it found before download, so you can check and correct the values. No PDF? Fill in the XRechnung directly in the form.

- [Convert invoice](/en/pdf-to-xrechnung)
- [Fill in XRechnung in the form](/en/create-xrechnung)

Trying it and buying a single conversion work without an account. Subscriptions from €10/month with annual billing (excl. taxes), or €5 per conversion (excl. taxes) without a subscription.

## Official sources and tools

- [KoSIT Validator](https://erechnungsvalidator.service-bw.de/)
- [XRechnung Standard](https://xeinkauf.de/xrechnung/)
- [Test Suite](https://github.com/itplr-kosit/xrechnung-testsuite)