A/P Invoice 
The A/P invoice is a request for payment. It also records the cost in the profit and loss statement.
To access the window, choose 
Purchasing – A/P 
A/P Invoice 
.
You can create an A/P invoice from multiple purchase orders and goods receipt POs. You cannot change any accounting-relevant data on an A/P invoice since it is the legal accounting document that generates entries in the general ledger.
When you receive an A/P invoice, SAP Business One posts the related accounts for the vendor in the accounting system. If no delivery for a purchase order precedes the A/P invoice, and if you are purchasing items managed in the warehouse, the inventory increases when the you post the invoice.
SAP Business One enables you to create an A/P invoice with a zero amount when you receive no-charge items, for example, items that are part of a promotion or under the coverage of a service contract.
Relevant in some localizations only:
If your company does not manage a perpetual inventory system, the A/P invoice is posted to the appropriate expense account: domestic, foreign, or EU.
Example
If the company is Danish and the federal tax ID of the business partner is also Danish, the invoice is domestic, so the system posts the invoice to the domestic expense account. If the federal tax ID does not start with DK (or is empty) and the billing address country of the business partner is an EU country, the system posts the invoice to the EU expense account. If the federal tax ID does not start with DK (or is empty) and the billing address is not EU, the system posts the invoice to the Non-EU expense account.
Country-Specific Information for Italy
When you create an invoice with a zero amount, note that you must charge VAT on free goods and that the customer has to pay the VAT.