In 19991, I started my professional career as a software tester at Microsoft. I began as a Software Test Engineer and then transitioned to a Software Design Engineer in Test. I also spent time as a Developer and Data Engineer, but I spent the vast majority of my time as a software tester. The oversimplification is that I verify ‘correct’ behavior. In my case, documentation and common sense define ‘correctness’. If the documentation defies common sense, I may push back to change the definition of ‘correctness’.
Occasionally, there are conflicting corner cases. For instance, children can ride in Space Mountain, but only if they’re at least a certain height. That’s just one rule. However, determining which rules apply under which conditions can be complex. There are safety and compliance standards (accessibility, for instance) that we may choose to adhere to.