In the context of the OWIN Framework a capability is an optional set of properties and methods that a middleware can choose to implement to make it more useful to the application developer.

None of the capability interfaces is necessary for proper operation, for example when middleware implements ISelfDocumenting it is possible for tools to programmatically extract documentation from the middleware which enhances the usability of the middleware for the application developer but it is always optional for the middleware developer to implement this or not.