Term used in customs administration. In line with the applicable customs regulations, the delivery of goods abroad requires an accompanying commercial invoice or pro-forma invoice on the basis of which the customs value of the shipment is calculated. A commercial invoice is created where the objects being transported have a commercial value. A pro-forma invoice is created when the items have no commercial value, for example if they are not intended for sale, if they are a gift, or in other similar cases.
The document must contain information about the consignor and consignee and above all, must contain a complete description of all contents along with details of their value.

Source: Zoll online, DPD, edited (only available in German)