This is the way I think of it.
Treats are unbalanced foods that do not provide all the nutrition that a complete diet does. They can be part of a balanced diet, but aren't balanced on their own.
Supplements are things that are very good for the chickens and provide nutrients that are either not in their base diet or are not provided in sufficient quantities.
Free range can provide a great many foods, but not always. The types and quantities vary quite a bit, from one location to another. All free range is not equal. The time of year makes a difference in most places, too. The amount of protein available in particular, can vary. A lush range can provide a large percentage of a chicken's diet. Even a poor range gives the chickens a chance to eat phytonutrients that are missing in their commercial diets. They can pick and choose what they need or want. Pastured chickens are a topic that is related to free range.
Commercial feed is balanced for most of the basic things a chicken needs. It doesn't provide the best diet for optimum health for the chicken or optimum quality meat or eggs. It lacks the phytonutrients found in greens, vegetables and fruits. It's also lacking in some of the important fatty acids. It's a lot better than a poorly put together diet by someone that doesn't understand a chicken's nutritional needs or the nutritional make up of all the different dietary ingredients.
Some people mix up their own dry feed. This can turn out better or worse than commercial feed, depending on what's in it.
A person can feed any combination of these things. It just depends on what's important to you and what you want to do. People have different situations, beliefs and goals.
Whenever you are adding to a chicken's diet and it's a large percentage of their caloric intake for the day, on a regular basis, then I think you need to be balancing the carbohydrates, proteins, fats, vitamins, minerals and providing a good calcium source. Some people feed a lot of fresh food, so that instead of it being a treat, it's a major portion of the diet. That can work great or poorly, depending on what they are feeding.