Articles in the site are typically structured hierarchically, so that you have folders, sub-folders and so on.

Any quizzes defined in a folder will incorporate all the questions of quizzes defined in sub-folders. These will be presented to the user in a randomized fashion.

This provides a great way to design your quizzes. Adding quizzes directly into articles where the information is available is incredibly convenient for the editor writing the quiz, but can be too easy for a quiz taker. 

So if, instead, the quizzes in the sub-folders are marked as When Can Take = Only When Requested, then it will be invisible to users in the sub-folder. However, you can then add a quiz at the top-level folder, and it will contain a roll-up of all the sub-folder quizzes. All of those will be collected together, and randomized before being presented to the user.

You will then have a far larger quiz they can take after they have read the entire section, and you will really get a clear idea of whether they have truly retained the information they read.