The warehouse's items. Each carries an SKU that is unique across the site.
Linking to the store is optional but decisive
An item may or may not be linked to a store product. Raw materials and supplies are never sold and need no link.
But if an item is not linked, selling that product in the store has no effect on this stock at all — it is skipped silently, with no error. This is the usual answer to "why didn't warehouse stock go down?"
On a variable product each variation gets its own item, so stock is kept per SKU rather than as one shared pool for the product.
When several warehouses hold stock, which one is drawn from?
Where an item holds stock in more than one warehouse, a store order is taken from the one with the most available stock. The store does not choose the warehouse.
A count correction is not an edit
When a physical count disagrees with the system, you do not overwrite the figure; you record an adjustment movement that makes up the difference, with its reason.
The result is that no figure ever changes unexplained — who, when and why can always be answered.
The reorder point drives suggestions
An item's reorder point and reorder quantity order nothing by themselves. They decide when the item appears on the Reorder page and how much is suggested.
The comparison is against available stock, not physical stock — so an item entirely reserved is suggested for reorder even with a full shelf.
SKUs are generated
Leave the SKU empty and it is built from the pattern set in Settings. An SKU is unique across the site, so two items cannot share one.