The predefined extension field framework allows you, as an SAP partner, to equip your SaaS applications with the support of customer-specific extension fields. Customers can extend partner applications with customer-specific extension fields once the application has been deployed on SAP BTP ABAP environment multitenancy.
An extensible partner application contains fields which are marked as predefined extension fields. Those fields are part of application but invisible in OData Services by default. You can enable the predefined custom fields in ABAP development tools for Eclipse and configure the extension fields using the Fiori app Configure Predefined Custom Fields.
Related Information
Working with Predefined Field Enablings