Metafields
The metafields are Namespaced Key - Value store for Apps.
The metafields can only be associated with the following entities:
- Product
- Product_Variant
- Category
- Page
- Order
- Customer
To do that you need to set the owner_resource to one of the above, an example would be owner_resource='Product'.
An use example would be use it in an app for Bookstores that require associate a book (product) with it's author and genre to provide an advanced search of books by this fields.