Part 1 of 3. CAT FAQ: Beginner

4. How does the GMAT select which questions I get?

CATs like the GMAT have a blueprint—a set of specifications (difficulty, question type, content area, etc.) that define which questions you see. At the same time, each question has certain statistical characteristics that the algorithm uses, based on your response, to estimate your quantitative or verbal ability. The algorithm looks at your performance on the questions you have already answered and the characteristics of each question remaining in the pool and then selects for you the question that simultaneously best satisfies the blueprint and provides the most statistical information it can, to generate the best estimate of your ability.
(Link: Part 1 of 3. CAT FAQ: Beginner)