Technical Requirements and Mandatory Data

🎯 Purpose

This guide explains the mandatory technical and data requirements for organizations integrating with Link4 to send and receive electronic invoices.

Before integration, ensure your ERP, Accounting System, or Middleware can supply all required data.

Missing or incorrect data will result in

1. invoice rejection
   
2. validation errors
   
3. delivery failures with authorities such as Peppol Europe or Peppol networks.
   

1️⃣ Supported Integration Method

API Integration

Your system connects to Link4 via a secure API.

Requirements

• API endpoints provided by Link4

• OAuth or API key Authentication

• Valid JSON payload structure
  

• Data validation compliance

2️⃣ Supported Transaction Types

Document Type	Description
Invoice	Standard commercial invoice
Credit Note	Reduces or cancels an invoice
Corrected Invoice	An updated invoice issued to correct errors in a previously sent invoice
Other	Additional document types by agreement

For all document types, the following data must be complete:

• Document header data
  

• Supplier and buyer details

• Line items

• Tax information
  

3️⃣ Contact Data Requirements (Buyers & Suppliers)

Accurate contact data is mandatory.

This data is used for:

• Addressing and routing (Peppol)

• Tax authority validation (e.g. EU Peppol)

• Delivery and notifications

3.1 Mandatory Fields (Per Contact)

Applies to BOTH customers and suppliers.

3.2 Optional

(Not specified in the original document.)

3.3 How to Provide Contact Data

Option A – API

• Each contact must be retrievable using a unique identifier

• All mandatory fields must be returned

• Used to avoid duplicates and ensure correct matching

Option B – CSV Upload

• CSV must include all mandatory fields

• A unique reference column may be required for create/update logic

• Column order and format will be confirmed by Link4

4️⃣ Tax Information

Tax data must comply with the target jurisdiction (e.g. EU Peppol, Peppol VAT rules).

4.1 Mandatory (Header Level)

• Supplier VAT ID

• Buyer VAT ID

• Tax exclusive/inclusive indicator

• Currency (EUR)

• Total net amount

• Total VAT amount

• Total gross amount

5️⃣Line Item Requirements

Each line must include:

Mandatory

• Line number

• Item description

• Quantity

• Unit of measure (EA, KG, HUR, etc.)

• Unit price

• Line extension amount

• VAT rate %

• VAT amount

• Line total

If VAT Exempt / Reverse Charge

• VAT exemption reason code

• Reverse charge indicator

• Legal text reference

Optional (Recommended)

• Seller item ID

• Buyer item ID

• GTIN / SKU

• Discount %

• Discount amount

• Order reference

• Delivery note reference

• Contract reference

6️⃣ Document Header Requirements

Mandatory

• Unique document number

• Issue date (YYYY-MM-DD)

• Document type (Invoice / Credit Note)

• Supplier details (VAT number, legal name, full address)

• Buyer details (VAT number, legal name, full address)

• Currency (EUR unless otherwise agreed)

• Tax-exclusive amount

• Total VAT amount

• Tax-inclusive amount

Optional

• Due date

• Order reference (PO number)

• Buyer reference

• Contract or project reference

• Payment terms

• Bank / payment details

• Delivery date

• VAT exemption reason (if applicable)

• Internal notes (must comply with Peppol format rules)

7️⃣ API Integration Requirements

Authentication

• Secure credentials (OAuth or API keys)

• Read and write permissions

Data Coverage

• Header data
  

• Line items

• Tax information

• Contact lookup via unique ID

System Behaviour

• No Duplicate documents

• Clear HTTP errors

• Stable identifiers for retries

8️⃣ Validation & Submission

All documents are validated against:

• Peppol BIS 3.0 schema

• EU VAT math consistency

• VIES VAT number validation

Missing or invalid data may result in:

• Document rejection

• Validation errors

• Delivery failure (e.g. buyer not on network)