Random Questions may be added to any category.
When you add a Random Question to a quiz, then it will be replaced with a randomly-chosen question from the same category - for each attempt.
This means that different students are likely to get a different selection of questions when they attempt this quiz. When a quiz allows multiple attempts for each student then each attempt will also contain a new selection of questions.
The same question will never appear twice in a quiz. If you include several Random Questions then different questions will always be chosen for each of them. If you mix Random Questions with non-random questions then the random questions will be chosen so that they do not duplicate one of the non-random questions.
The grade for the randomly chosen question will be rescaled so that the maximum grade is what you have chosen as the grade for the Random Question.