What is the default behavior with and without router option?

[jujoco]

The README.md doesn't explain or describe the default behavior of the proxy when the a router match is not found.
it just says Re-target option.target for specific requests. my question is, when a router match is not found will it default and fallback to the provided target? I was stuck on trying to understand the concept of the router property because i was clearly having to pass a target. I don't want to assume any behavior. if someone can just comfirm this. and maybe explain a use case for using the router property I would appreciate that so much!
Thank you in advance!