Great question! Study.com offers courses with a placement test option if you have taken a placement test for your course, any lessons that were mastered, will be marked optional or completed. If your course does not offer a placement test and you see optional lessons, or completed lessons that you have not yet visited, this is due to our knowledge check feature. This knowledge check feature is similar in that you can achieve mastery. Placement tests offer mastery for the full course concepts whereas, our knowledge check feature masters subjects on a quiz-by-quiz basis.