Suppose that I have a set of axioms in first-order logic. And suppose that I have several inequivalent models for this set of axioms. And suppose that I want to choose one specific model. To choose it, I need to make some additional claims which specify my model uniquely. My question is the following: What kind of claims these additional claims are? Are they some additional axioms? Or are they claims which are not classified as axioms? If they are not classified as axioms, what property do they have/lack so that they cannot be classified as axioms? If the question looks too abstract, let me consider an example. Suppose that I start from axioms of group theory. There are many different groups satisfying these axioms. So I choose some specific group, say SO(3), defined by some claims which define that group. Are these claims also axioms? If not, then what property do they have/lack so that they cannot be called axioms? Could it be that my confusion stems from the fact that the word "axiom" in logic has more narrow meaning than the word "axiom" in the rest of mathematics?