In Customizing, using Block transfer of requirements by delivery block, you can specify that certain delivery blocks in the sales document header also block the confirmation of order quantities.

If you enter such a delivery block in a sales order header, the system confirms the available quantity to show you the delivery options. If you save the document, however, the confirmed quantities are not saved. Your company could block confirmation for a customer if this customer's credit worthiness was questionable. The sales order in this case is not only classed as unconfirmed, it is also blocked for delivery.

Thus, no stock is reserved. The stock is still available for other customers. Although the order quantities are not confirmed, the system still transfers the requirements to material requirements planning. It is assumed that the delivery block will be removed at some point. The automatic transfer of requirements means that the order quantity can be confirmed at a later point in time.