A person may conceive “a mountain of ruby” even without ever encountering such a mountain. Having its meaning in the mind and finding a corresponding thing outside the mind are distinct matters.
Similarly, mentioning an impossible description does not establish an external instance of it. The description can be considered, but existence is a further question requiring evidence.
This matters because universal versus particular is determined by the structure of a meaning, while existence or nonexistence concerns what reality and argument establish.