The phrase 'cassus omissus' has a misspelling. The correct phrase is 'casus omissus', which means 'a case left out'. The second phrase, 'expressio unius est exclusio alterius', which means 'the expression of one is the exclusion of the other'. So the first phrase refers to an omission, and the second to two mutually exclusive alternatives.

The Latin phrase "expressio unius est exclusion alterius" means that the expression of one thing excludes the other. In legal contexts, it suggests that when something is explicitly mentioned, it can be assumed that other similar things are intentionally excluded from the provision.

Literally the sentence means: affirmation of one [reality] is the exclusion of its alternative.

The word "alternative" comes from the Latin root of alter (nominative) alterius (genitive) which is the other of two. This expression must therefore refer to the fact that in arguing or discussing an issue with only two possible solutions, the evidence proving the one [solution] eliminates the veracity of its alternative or second possible solution. This is my guess. Frankly I never heard of this expression before, and I am an old man.

inclusio unis est exclusio alterius

The Ninth Amendment was enacted to ensure that a particular rule of statutory interpretation is not applied: expressio unius est exclusio alterius (which means something like, the expression of one is the exclusion of all others). Without the Ninth Amendment, people could try to argue that because the Bill of Rights lists certain rights of the People, those are our onlyrights.

Although some people have looked to the Ninth Amendment as a way for the federal courts to recognize individual rights not explicitly mentioned in the Constitution (under a theory that there are basic human rights or that there are certain god given rights), the Supreme Court has been extremely reticent to do so. Off the top of my head, I can't think of any substantive rights justified using the Ninth Amendment.