To decide what to purchase and manufacture use the forecasting and current demand dashboard
Forecasting uses the sell rate of the product, the current stock levels and any expected stock to show how much will be available over time. The forecast is sensitive to the delivery time expected (by supplier or by individual item).
See Perishable item Simple Forecasting and Perishable Item Complex Forecasting
The Current Demand Dashboard lists the articles that are on sales orders that are not available in stock at the moment.