A metafield definition describes a reusable custom field: where it applies, its namespace and key, its value type, and the rules values must satisfy. A metafield value is the data attached to one product, variant, customer, order, order item, collection, category, page, blog item, vendor, brand, or the store.
Namespace and key form the stable name integrations and themes use. The owner type, namespace, key, and value type are locked after creation so existing values and integrations cannot silently change meaning.
Merchant definitions and app definitions have separate allowances. An app can manage only definitions in its own namespace. App access settings control whether merchants or other apps can read or edit app-owned data.