There isn’t a clear one because it’s not simple to define. If you follow the research, we can’t simply say it’s the ingredients used as the process itself can be bad. So it’s a hodge-podge of, “well watch out for this ingredient, but also ask them if they’ve put it through x and then we’ll know”.
It’s a whole area of research and it’s going to remain complicated, but we can’t give up on describing it at all.
Ah brilliant! Case closed. Pack up everyone.
Look, there’s plenty of research indicating very negative effects of UPF on us, but it’s still an early field. The categorisation of UPF isn’t perfect no question, but that’s largely a result of the fact that, cooking processes combined with ingredients don’t make nice categories. We need ways to simplify the research for people who realistically aren’t going to look into the data, such as yourself.
If you’re really that opposed to the field, like your dismissive tone implies, then I’d advise not researching it, and eating what you like. But you aren’t going to erase the fact that obesity rises as exposure to UPF rises, that food preparation alters how our bodies process it, or that there are different health outcomes especially if you eat a lot of it. I’m afraid that’s science.