# BT-128 Invoice line object identifier

An identifier for an object on which the invoice line is based, such as an order line, despatch advice line, or tender line.

## Quick answer

An identifier for an object on which the invoice line is based, such as an order line, despatch advice line, or tender line.

> **Line items**: The goods or services on each invoice line, including quantities, prices, and VAT.

## In plain language

An identifier for an object on which the invoice line is based, such as an order line, despatch advice line, or tender line.

An identifier for an object on which the invoice line is based, such as an order line, despatch advice line, or tender line.

Links the invoice line to the corresponding line in the source document (order, tender, etc.).

## What you usually enter here

Alphanumeric identifier with meaningful reference

Typical valid values include: PO-001-1, ORDER-123-5, TENDER-ABC-2

Optional but recommended for traceability and audit purposes.

## What goes wrong often

Avoid values like: , reference

Relevant rule codes: BR-2

## Examples in context

Links the invoice line to the corresponding line in the source document (order, tender, etc.).

Optional but recommended for traceability and audit purposes.

## Valid Examples

- PO-001-1
- ORDER-123-5
- TENDER-ABC-2

## Invalid Examples

- reference

## Related Fields

Links the invoice line to the corresponding line in the source document (order, tender, etc.).

- [BT-126 Invoice line identifier](/resources/xrechnung/bt-126-invoice-line-identifier)
- [BT-13 Purchase Order Reference](/resources/xrechnung/bt-13-purchase-order-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-128

### Data Type:
identifier

### Cardinality:
0..1

### Requirement:
Optional

### Max Length:
50 characters

### Since Version:
Optional

## XML Implementation

### UBL 2.1 Syntax
```
/Invoice/cac:InvoiceLine/cac:DocumentReference/cbc:ID
```

### CII (UN/CEFACT) Syntax
```
/CrossIndustryInvoice/SupplyChainTradeTransaction/ApplicableHeaderTradeSettlement/ram:SpecifiedTradeSettlementLineMonetarySummation/ram:AssociatedDocumentLineDocument/ram:LineID
```

## Validation Rules

### BR-2
BR-2

## Why the standard cares about this field

### Business Context
Links the invoice line to the corresponding line in the source document (order, tender, etc.).

### Legal Requirement
Optional but recommended for traceability and audit purposes.

## 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)