# Purchase orders/purchase invoices

Source: https://developer.avalara.com/vat-erp/transactions/certification-requirements/purchase-orders-and-invoices/

**Element type:** Required for Input VAT, Suggested for Core

**Endpoints/models used:** [CreateTransactionModel](https://developer.avalara.com/api-reference/avatax/rest/v2/models/CreateTransactionModel/)

Your integration and relevant document types must mirror your ERP system workflow for a transaction lifecycle when creating purchase orders and purchase invoices. For example, in a workflow, before payment has been process and before the document is committed, the transaction quote should call the `purchaseOrder` doc type. After the order is turned into an invoice, then change the document type to `purchaseInvoice`.