The inventory surfaces — stock levels, purchase orders, sales orders, shipments — appear only when you sell physical goods. A product's kind can be a service, a subscription, or a good; marking one as a good (and enabling stock tracking) turns on the inventory engine. A services-only business never sees these surfaces.

Stock comes in on a purchase order from a vendor and is received into a location. When a customer accepts a goods quote, a sales order is created so fulfillment can begin; you ship against it, drawing stock down and recording a shipment. Every stock movement is logged, so on-hand, allocated, and available quantities always reconcile.