In a traditional classroom, students might demonstrate mastery of a concept by taking a test. Or maybe the whole class writes reports. Perhaps the teacher assigns a public speaking exercise. Any one of those exercises surely demonstrates mastery by some students, while potentially leaving out others.
In a personalized learning environment, each individual student can work with their instructors to develop ways to demonstrate mastery in ways that make sense for them. Gone are the days of one-size-fits-all assessments that work for few.