Almost all people use mathematics where "Axiom of Choice" is set to be true. It is grossly-unusual to consider systems without that axiom.
If you are not a mathematician chances are you've never encountered a system based on axioms without "Axiom of Choice" (for systems where it is applicable at all).
If you are not a mathematician chances are you've never encountered a system based on axioms without "Axiom of Choice" (for systems where it is applicable at all).