HI @AndreasKleinfeld72 

According to the help, the document type is either going to be a supply or demand. Both a sales order and shipment are a demand. 

Demand documents are documents that generate demand for items and cause items to be issued from one of the company’s warehouses. The demand document may be a sales order, a forecast, or a production order for materials included in an item to be produced. Supply documents are documents that supply items required in production and cause receiving items in one of the company’s warehouses. The supply document may be a purchase order or a production order for an item to be produced.

The system also considers the other data stored in the system that may affect item availability. The extended list of documents and data used in MRP is the following:

• Forecasts
• Sales orders that are not processed until completion, including open transfer orders and blanket sales orders
• Shipments with not issued items
• Inventory on hand
• MPS orders
• Production orders that are not processed till completion
• Purchase orders that are not processed till completion

The source also appears to have an effect on the type. What is the source of the order? 

Is there a shipment record that is not complete - if so, shipments can generate demand