moodle/lang/en/help/quiz/random.html

<p align="center"><b>Random Question</b></p>

<p>Random questions are a special question type.</p>

<p>When you put a Random Question into a quiz, then a question 
   will be chosen randomly from the whole category, on each attempt.</p>

<p>The maximum grade for the question will always be when you have chosen
   as the grade for the Random Question.</p>

<p>If you make a quiz with, say, 10 random questions, then each student 
   may get a completely different set of 10 questions each time they
   attempt the quiz.</p>

<p>Note that you can mix random and non-random questions if you want to 
   ensure that particular questions are always included.</p>