Kantian maxims are a kind of subjective rules providing rationale or justification for concrete ethical choices. A proper Kantian maxim should be a function from a list of conditions and a motive or aim to a uniquely determined conclusion that a particular concrete choice is or is not permissible for a moral being. It does not tell us exactly what to do, but it is expected to definitively tell us whether something is okay or not okay. It is a kind of inference rule.
Many maxims will fail to be universalizable. Kant says we should only trust the ones that can pass testing by the categorical imperative.
Where Aristotle had stressed an open-ended rational inquiry and the irreducibility of ethics to an exact science, Kant recommended focusing deliberation more narrowly on a search for deterministic functions satisfying the categorical imperative that will tell us if possible actions are okay or not.
Another important difference is that Kantian deliberation stops at what is permissible, whereas Aristotelian deliberation extends all the way to what to do, so the Aristotelian kind has a strictly broader scope.
The question is whether by thus narrowing the scope of ethical inquiry in conjunction with his other moves, Kant really succeeded in making the narrowed scope fully deterministic.
Kant talked much more about testing maxims than about searching for them or formulating them. If we were searching, presumably we would try to match on the conditions and aim that would be the inputs to the function. There might be questions about the granularity with which the conditions and aim are specified. To adequately address the complexities of real life, we would need a huge array of possible functions.
It is hard to even imagine a procedure for initially formulating the function-body of a maxim that would tell us specifically how to get from the inputs to a deterministic output. All we have is tests whether an already formulated candidate maxim is universalizable or not. Actual formulation of maxims thus seems to be left to trial and error. Kant might say the important thing is the ability to test, but it seems to me that if we cannot deterministically say how maxims are to be formulated, we cannot really claim to have a deterministic solution to the whole problem of ethical decision-making.
It seems as though Kant was successful in establishing that valid ethical conclusions do have necessary conditions that no one before him recognized, but unsuccessful in defining conditions that would be both necessary and sufficient to derive those conclusions, even at the level of just considering what is permissible. Thus, we still need Aristotelian open-ended deliberation and practical judgment, or an ethical analogue of Kant’s own notion of free play in aesthetics. I also still like the Leibnizian principle of wise charity — within reason, doing more and demanding less than what is nominally required of us.