The phrase "to mark off" means to indicate or identify something by drawing a line, writing, or making some other kind of physical or visual indication. This can be done with pen and paper, using chalk on a blackboard, or even physically marking out an area with tape or paint. The purpose of "marking off" is to clearly define the boundaries or limits of something, such as drawing lines on a map to show where one country ends and another begins, or highlighting text in a book to indicate important information. In some cases, it can also mean to separate or distinguish something from others by giving it special treatment or recognition.