Documents Data source in SAP B1 Inventory Transfer:

The Documents Data Source step specifies the types of documents that are used to calculate supply and demand for products, such as production orders, sales orders, purchase orders, and identifies which inventory level to consider. Here you choose whether to consider a sales forecast. If you ran the MRP wizard by warehouse, you can choose to generate the document to the item’s default warehouse, or to the warehouse where the demand is triggered from. You can also add the option to recommend Inventory transfer requests between warehouses in case there is sufficient inventory in another warehouse location.