# BT-18 Invoiced object identifier

Identifier of the project, asset, case, or other business object the invoice belongs to.

## Quick answer

Identifier of the project, asset, case, or other business object the invoice belongs to.

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

## In plain language

Identifier of the project, asset, case, or other business object the invoice belongs to.

Identifier of the project, asset, case, or other business object the invoice belongs to.

Use this when invoice processing is organized around a non-order object such as a project ID, case ID, or asset number.

## What you usually enter here

Business object identifier

Typical valid values include: PROJECT-7784

Optional according to EN 16931-1.

## What goes wrong often

Avoid values like: , AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

## Examples in context

Use this when invoice processing is organized around a non-order object such as a project ID, case ID, or asset number.

Optional according to EN 16931-1.

## Valid Examples

- PROJECT-7784

## Invalid Examples

- AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

## Related Fields

Use this when invoice processing is organized around a non-order object such as a project ID, case ID, or asset number.

- [BT-11 Project Reference](/resources/xrechnung/bt-11-project-reference)
- [BT-19 Buyer accounting reference](/resources/xrechnung/bt-19-buyer-accounting-reference)

## 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-18

### Data Type:
identifier

### Cardinality:
0..1

### Requirement:
Optional

### Max Length:
50 characters

### Since Version:
Optional

## XML Implementation

### UBL 2.1 Syntax
```
/Invoice/cac:AdditionalDocumentReference[cbc:DocumentTypeCode="130"]/cbc:ID
```

### CII (UN/CEFACT) Syntax
```
/CrossIndustryInvoice/SupplyChainTradeTransaction/ApplicableHeaderTradeAgreement/ram:AdditionalReferencedDocument/ram:IssuerAssignedID
```

## Why the standard cares about this field

### Business Context
Use this when invoice processing is organized around a non-order object such as a project ID, case ID, or asset number.

### Legal Requirement
Optional according to EN 16931-1.

## 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)