When a lesson contains one or more Branch Tables, you should set this parameter. Its value sets a lower limit on the number of questions seen when a grade is calculated. It does not force students to answer that many questions in the lesson.
For example, setting this parameter to 20 will ensure that grades are given as though the students have seen at least this number of questions. If a student only looks at a single branch in a lesson with five pages, answers all the associated questions correctly and then chooses to end the lesson, their grade would be 5 out of 5 if this parameter were left unset. Setting the parameter to 20 ensures the student would instead receive a grade of 5 out of 20.