metrics
collective user feedback/demand
person-in-power feedback
generally own work prioritisation is expected from mid level employee
team level prioritisation is expected from senior level employee
prioritisation is viewed as responsibility from leadership, while as a skill by entry level employees
don't let yourself intimidated by lack of proof
generally, if prioritisation is not lead by metric or user study, then it is just about your level (which is what you negotiated and it is not about the skill)