Metafield

Products, variants, orders, locations and customers can all carry them.

What stockrooms use them for

• The bin location a variant lives in
• A supplier’s own product code, next to your SKU
• A case quantity, so a scan of an outer counts as twelve
• The date a batch expires

Shopify has no field for any of these, and every one of them is something a stockroom needs on the screen.

Define them properly

Create a definition first, with a name and a type, rather than writing loose values. A defined metafield is validated, appears in the admin as a proper field, and can be edited by a person instead of only by an app.

Where it goes wrong

An app writes its data into its own private metafields and does not document them. Uninstall the app and the values are still there, unreadable, or they are deleted with it. Ask where an app keeps its data before you depend on it, and check you can export it through the Admin API.

The other one is using a metafield as a stock count. Quantities belong on the inventory item, where Shopify updates them when an order is placed. A number in a metafield is a number nothing keeps true.