# 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

Code that classifies the subject of an invoice note.

Code that classifies the subject of an invoice note.

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.

## What goes wrong often

Avoid values like: note

## Examples in context

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

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

## Valid Examples

- AAI

## Invalid Examples

- note

## Related Fields

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

- [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

### Since Version:
Optional

## 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
```
/CrossIndustryInvoice/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)

Download requires an account. Subscriptions from €10/month with annual billing (excl. taxes), or €5 per conversion (excl. taxes) without a subscription.

## Why the standard cares about this field

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

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

## 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)