Mixing. If it sounds good, it IS good.
…and that’s really the only rule.
A quality mix engineer will find the heart of a song and make it shine, while refining the supporting elements and giving greater context to the overall production.
Sometimes it means leaving things raw and untreated. Sometimes it means cleaning and tightening to perfection. Or maybe it’s getting crazy with some granular delays and retro wobble effects.
Maybe it’s all 3…