unit testing (and any kind of testing for that matter) shows that your code works for a set of inputs. formal verification methods show that it works for all in

unit testing (and any kind of testing for that matter) shows that your code works for a set of inputs. formal verification methods show that it works for all inputs. it’s the holy grail but comes with a really high price tag to actually do it (so unless you’re writing something critical or have a lot of resources you normally get your software to the point where it’s good enough through unit/etc testing)