Answer to Question 1
Achievement tests measure skills and knowledge in specific areas. They are used to measure learning that has occurred. Aptitude tests measure general abilities such as reasoning, perceiving relationships, analogies, etc. They are used to predict success, especially school success.
Answer to Question 2
It is a restriction in range at the upper end of a score distribution that prevents individuals from performing to their maximum ability. It is important to consider in research because the effect of a treatment could be underestimated simply because the test does not measure achievement accurately at the upper end of the range.