what is X-Cascade header

Rack documentation says that

File servers support X-Cascade header

rack-mount says that

Rack::Mount supports Rack’s +X-Cascade+ convention

What is X-Cascade ? I could not find any documentation on X-Cascad开发者_开发知识库e.

The server will set the X-Cascade header to "pass" to continue to try other routes. This allows for multiple routes to be nested/stacked.

The general purpose of this is to pass the request on to other middlewares to handle the request if that specific handler doesn't handle it.