Custom Fields allows you to edit any Metafield, even if it wasn't created by our app. This is provided for convenience and to ease any transition from another Metafield app to ours.
We always recommend using our structured field types rather than "raw" Metafields, but sometimes that's not practical.
Click on Edit Custom Fields on any product, then scroll down to the bottom of the page where you will find a section called Other Metafields. Open it up to reveal any other metafields attached to your product.
Additional Resources and Information
Custom Fields: Using Custom Fields Beyond Products - This section goes through different uses of Custom Fields other than product pages.