> A good example is validation.
It is a good example to bring up because front end validation is not business logic, it is presentation logic. For some simple use cases that might not be obvious to many people but let's just consider a sign up form accepting an email address and a password. There definitely validations you want to perform in both places - the password needs to be 10 characters and have at least 3 of numbers, uppercase, lowercase, and special characters. Those exact password validations will be the same in both the UI and API.
But let's consider the email. The business rule here is that only deliverable emails that are openable by the person signing up should be accepted for new users. You cant validate that on the front end. The best you can do is check for an @ symbol or run a more complicated regex (whales be there) before passing it off to the backend to actually send an email, encode signup details into a special link, and do all the work necessary to satisfy the business rule.
Its trivial to further drive that point home that UI validations are not business logic even when the same validation appears as part of business logic. You do some minimal checks to save the user time on the UI, but those shouldn't even be looked at as validations. They should be thought of as affordable for the user to prevent wasting their time with trivial, predictable errors.
