Micellar casein is a dairy protein that digests slowly because it forms a gel in the stomach. Ultrafiltration strips away fat and lactose while keeping the natural micelle structure intact. That gel matrix releases amino acids steadily over several hours, unlike the quick spike you get from whey.
People use it to fuel muscles through long stretches without food, typically overnight or as a meal replacement when extended fullness is the goal. A shake before bed is the common habit. Because of its slow absorption curve, it rarely shows up in post-workout shakes.
Beware if you have lactose intolerance or any dairy sensitivity. Anyone with a true milk protein allergy should avoid it entirely.