How it works
On hand is what is physically in the location. Committed, or reserved, is what open orders have claimed. Available is on hand minus committed, sometimes minus safety stock. Each system calculates these slightly differently and updates them at different speeds, which is how overselling happens when several channels sell the same stock.
Example
A warehouse holds 120 units of a SKU, 35 are reserved for open orders, so 85 are available. Shopify, Amazon and the wholesale portal should all show 85 or less.
In integrations
The Unify Inventory API is designed to split quantities the same way across ERPs, warehouse systems and storefronts, by location, and to update stock with adjustments rather than overwrites so concurrent changes do not clobber each other. Change events fire when quantities move. Overselling is expensive: cancellations, marketplace penalties and lost customers. Accurate, fast-moving inventory data across channels is the main defense, and it starts with consistent quantity definitions.