While creating a test, you can choose to add a random block of questions, and/or a question set. Both features enable you to add a group of questions all at once. However, they meet different teaching needs.
Before we talk about the difference between a question set and random block, let's look at the similarities:
Both consist of a group of questions that you specify
In both cases, when you add them to a test, you specify how many of the questions from the group the test should display
Both give you an easy way to add a specific group of questions to a test, to randomize questions, and to reuse that group of questions on other tests
What is the difference then? Simply put, the main difference between random blocks and question sets, is the source of their questions.