The defining characteristic is usually direct oversight/power. Can the person reward/punish/fire you? If so then there are ethical concerns involved in a relationship (although a relationship may not be strictly forbidden). Is the person an authority figure like a police officer? Same deal.

Relationships are complex enough that judgments should be, and to the best of my knowledge often are, made on a case-by-case basis. E.g. of course police officers can still have relationships, but they can’t say “Date me if you don’t want a ticket”. For bosses working every day with employees it’s also tricky. In the professional circles I know of it’s considered risky for a boss to be sleeping with an employee but not forbidden. What is outright illegal is pressuring an employee into a relationship of any kind. Sexual/romantic relationships can still happen, but consent has to be clearly stated and unforced.