While both pure milk and breakfast milk contain milk, pure milk is just milk, whereas breakfast milk is formulated to replace an entire meal.
First, the ingredients of breakfast milk and pure milk are different. The ingredient list for pure milk contains only three words: fresh milk. The ingredients for breakfast milk are more complex, including milk, water, white sugar, malt extract, peanuts, egg powder, oats, and possibly stabilizers, iron fortifiers, zinc fortifiers, and flavorings.
Secondly, their nutritional values also differ. The protein content of pure milk is 2.9% to 3.1%, while breakfast milk is above 2.3%. It's clear that after adding sugar and other substances, the protein content decreases slightly. However, because other ingredients like eggs, wheat, and peanuts also contain a certain amount of protein, the overall reduction is not significant. If the protein content is labeled as ≥1.0g, it is considered a milk beverage rather than milk, and its nutritional value is naturally relatively lower.
Pure milk is a food with quite rich nutrition, but it is not perfect. First, if used as a staple food, its proportion of carbohydrates is low, so it's best to pair it with some starchy foods. At the same time, milk's natural deficiencies are its low content of iron and zinc, and too little vitamin C. Therefore, it's suitable to pair it with some nuts, plus a little fruit or vegetable. A cup of milk, two slices of bread, a small handful of nuts, and a fruit like a kiwi make for a nearly perfect breakfast. If you have plenty of time for breakfast, drinking milk with these foods is, of course, the ideal approach.
But if time is tight and you can't have breakfast? In that case, drinking just a carton of milk is certainly better than nothing, but it's not nutritionally complete. If you switch to drinking breakfast milk, its nutrients such as carbohydrates, fats, and protein are more in line with the requirements of a meal.