Many things are clearly Halal or clearly Haram. There are some things that are still indistinguishable and more information is needed to know it’s status. These items are often referred to as Mashbooh, which means “doubtful” or “questionable”. 

It’s a very long list. Let us take  few examples of food additive the most common thing that we usually encounter in our daily lives, as  Food additives such as gelatin, enzymes, emulsifiers, Fat (Edible Oils  except it is purely source of veg. or Pure Ghee derived from halal mammals fat) etc  can be derived from the source of animal or plants. The question is, if the source is from an animal origin, is the animal being slaughtered according to the Islamic law (Zabiha)? If it is not Zabiha, then the food containing these additives would be Haram too.