The ostensible purpose of the Ninth was to place the presumptive burden on the Constitution to prove that it was the repository of rights, rather than on The People to prove that they were.
It was never expected that the Constitution would provide an exhaustive list of the rights of a free people. But the absence of a certain right did not mean that it didn't exist. The PRESENCE of a right (an "enumerated right") put that right under the domain of the Constitution. But rights could still exist whether they were explicitly mentioned in the Bill of Rights. And those rights were reserved to The People.
I don't know what "reserved to" means but "retained by" means the people have them and can see and say what they have. The people are the ones who say what "unenumerated" rights they have.
Some people think the Ninth is an expansion or the federal powers in the 14th Amendment. Just the opposite of what was intended.
The Ninth says the federal courts must heed the people.
James Madison: ''It has been objected also against a bill of rights, that, by enumerating particular exceptions to the grant of power, it would disparage those rights which were not placed in that enumeration; and it might follow by implication, that those rights which were not singled out, were intended to be assigned into the hands of the General Government, and were consequently insecure. This is one of the most plausible arguments I have ever heard against the admission of a bill of rights into this system; but, I conceive, that it may be guarded against. I have attempted it, as gentlemen may see by turning to the last clause of the fourth resolution [Ninth Amendment].''