Modal logic extends ordinary logic with operators for necessity and possibility, but "modal logic" isn't one single formal system with one fixed set of rules; it's a whole family of related systems, each built by adding a different specific set of axioms governing how necessity and possibility are actually allowed to behave. Two of these systems can genuinely disagree about whether a particular modal inference is valid, which means choosing which system actually fits a given application, in ethics, computer science or elsewhere, means explicitly choosing which of these competing axioms should hold.
Different axioms encode different assumptions about what necessity actually implies
The weakest common modal system accepts only the most minimal assumptions about how necessity behaves, while stronger systems add further axioms, one holding that whatever is necessary is also actually true, another holding that if something is necessary, it's necessarily necessary, an assumption about the underlying structure connecting different possible worlds to each other. Each additional axiom rules out certain patterns of reasoning the weaker system would have permitted, which means the choice of system genuinely changes which modal arguments the resulting logic will actually validate.
The right system to use depends entirely on what "necessity" is meant to represent
A modal system built to reason about logical or mathematical necessity, what must be true in absolutely every conceivable circumstance, typically needs a considerably stronger, more restrictive set of axioms than a modal system built to reason about, say, a computer program's possible future states, where "possible" means something considerably weaker and more permissive. This is exactly why modal logic's family of competing systems isn't philosophical indecision so much as a genuinely necessary flexibility: different real applications need necessity and possibility to behave according to meaningfully different rules, and no single system correctly serves every one of those applications at once.
What we're still unsure about
That modal logic comprises a genuine family of distinct systems, each validating different patterns of necessity-and-possibility reasoning, is well established, extensively developed logical machinery. What's more genuinely an ongoing matter of philosophical and technical judgement is exactly which system correctly captures a given intuitive notion of necessity for a specific real application, since reasonable logicians and philosophers continue to disagree about which axioms genuinely belong in, say, the correct logic of logical necessity versus the correct logic of moral obligation, and that choice continues to be actively debated and applied differently across different fields rather than being settled by one universally correct modal system.
This sits inside Modal Logic, one of seven topics in Logic, one of five domains in Philosophy, one of seventeen subjects the app can quiz you on.