# BT-21 Invoice note subject code

Code that classifies the subject of an invoice note.

## Quick answer

Code that classifies the subject of an invoice note.

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

## 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
```

## 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/)
- [Invoice Converter](/pdf-to-xrechnung)
- [XRechnung Standard](https://xeinkauf.de/xrechnung/)
- [Test Suite](https://github.com/itplr-kosit/xrechnung-testsuite)