The Ed-Fi Core XSD defines a base set of data elements, attributes, types and structures. The Core XSD is an expression of the Ed-Fi Unifying Data Model as an XML Schema Definition file. It forms the base for and is referenced in all Ed-Fi Standard Interchange Schema – and can be used by developers to compose custom XML interchange schema for the exchange of information in their own apps.
The download link on this page contains just the "bare" Core XSD, which is usually used as a starting place for custom XML interchanges. If you're looking for a schema to handle a specific common data exchange scenario, have a look at the Standard Interchange Schema Documentation to see if your use-case is already handled by one of the existing standard XSD documents.