WinFS Types and Extensibility
WinFS's type system is tied to the .NET Framework's concept of classes and inheritance.
WinFS Type Extensibility allows developers to extend WinFS by providing a schema that defines new data types and associations.
These schemas are XML files, though they are not W3C XML Schema Definition, presumably because there are incompatibilities between base types (e.g. WinFS's "integer" data type is nullable, whereas XSD's integer type is not).
Custom schemas can be installed on a per-application basis (in situations where an application wants to store its data in WinFS, but not share the structure of that data with other applications), or they can be made available across the system.