Product Availability Rules
Product Availability Rules allow you to control when a product is available for purchase on your website and how that availability is communicated to your customers.
Traditionally, many platforms simply expected that if your quantity on hand was 0, the product could not be purchased, if QoH was more than zero it could be purchased. However, as the market place changes it is rare for any vendor to actively stock all items they sell. Some items may be stocked locally, some items available at a local distributor, some items at a distant distributor, some items dropped shipped from the manufacturer, etc.
For this array of availability levels you might want to present some products as "In Stock", some as "Ships in 3-5 days" and others as "Ships in 7 business days". Managing customer expectations is part of keeping your customers satisfied and providing realistic product delivery estimations is part of that.