In mandatory reference setting in the sales order, which of the following can you use?

In SAP SD mandatory reference configuration, valid reference documents for sales orders include Sales orders (for returns or subsequent orders), Inquiries (converted to orders), Quotations (accepted quotations), and Billing documents (for returns or corrections). Delivery documents cannot serve as mandatory references for sales order creation - the relationship flows the other way (sales order → delivery). The mandatory reference setting prevents document creation without a preceding document in the sales cycle.