# Invoices
This section will guide you through the core principles on how to us invoices within Procurement.
# Object Graph
┌─────────────────┐ ┌─────────────────┐
│ Contract │ │DeliveryLineItems│
└─────────────────┘ └─────────────────┘
▲ ▲ Optional
│ │
│ │
┌─────────────────┐ ┌─────────────────┐
│ Invoice │◀─────│InvoiceLineItems │
└─────────────────┘ └─────────────────┘
# General Concept
The invoice feature within procurement is a simplification of a regular invoicing system and is not meant to replace proper invoices.
Instead it allows your suppliers to inform you about the content of inbound invoices. On top, because the Procurement platforms presents invoices in a well definite structure via API, it allows you to analyse invoices and to optimize future orders.