The Armed Services Vocational Aptitude Battery (ASVAB) is used by the US military to evaluate applicants. It has two purposes. First, it is used to determine eligibility; you must achieve a certain minimum score to enlist in each branch. Second, your scores are used to determine your aptitude for certain types of work (“vocations,” hence the name). Therefore, you must not only achieve the minimum score to enter your desired branch, but you must also perform well enough to be assigned to certain careers in the military.
The ASVAB dates back to what was arguably the first modern standardized test: the “Army Alpha” test used to assess 1.7 million recruits in World War I. Since then, it has been the subject of intense scientific research because of its importance both to the military and to all the individuals that take it. The CAT-ASVAB refers to the most recent version, which is administered in a CAT (computerized adaptive test) format. For a technical introduction to the CAT-ASVAB, please read the article below.
Pommerich, M., Segall, D.O., & Moreno, K.E. (2009). The nine lives of CAT-ASVAB: Innovations and revelations. In D.J. Weiss (Ed.), Proceedings of the 2009 GMAC Conference on Computerized Adaptive Testing.