NUnit Extensibility is a mechanism developed by the NUnit team for use in extending the NUnit engine, runners, test adapters and other components. It may also be used independently from NUnit as a general extensibility approach.
The NUnit.Extensibility package provides the implementation of the extensibility mechanism. It is used by hosting assemblies to identify and load the extensions they support.
The NUnit.Extensibility.Api package provides the interfaces and attributes used to define extension points and extensions. It allows hosting assemblies and extension assemblies to interoperate successfully.