While your Products act as the overarching templates for what you buy and sell, and Materials define the raw botanical species, Items represent the actual, physical batches of goods that are shipped and tracked in each delivery.
Think of it this way: "Sawn Timber" is the Product in your catalog, but the specific 500 kg bundle of Sawn Timber that left your sawmill on Tuesday is the Item. Tracking at the item level gives you granular visibility into exact quantities, batch numbers, and the real-time physical whereabouts of your inventory.