This article referes to Metafields Editor app and not the Metafields Pro app.
There are 3 plans for Metafields Editor.
Free: These are for users who signed up when the application was free (for 8+ years!). Due to the increasing demand on the resources needed for an ever increasing user base. It was no longer viable to offer the app for free.
Basic: This gives you access to metafields for all resource types in Shopify: Product, Variants, Collections. Customers, Orders, Blog, Articles, and Shop. Bulk import/export is done via a credit system. See more about the credit system here.
Pro/Unlimited: This include everything in the Basic plan. You do bulk import / export with no restrictions.