There are no absolutes, every college is different. The B.A.S. has become fairly common recently. Most often is is targeted at students who have a A.S. or A.A.S degree in a technical or applied area. It is often called an "inverted major" or inverted degree. Students have already completed all or most of a major in a 2-year college program. However, the general education requirements are much less. Students complete the panoply of traditional education requirements in the final two years. Common areas of study are engineering technology, information technology, management, health sciences, etc. The most simplistic answer is that persons holding a 2-year college degree designed to prepare the graduate for immediate employment complete the requirements for a 4 year degree. Thus, the inverted language. Much of the coursework taken during the first two years in the B.S. is in the final two years of the B.A.S., and conversely many major courses taken in the last two years of the B.S. were taken as part of the degree, the A.S., A.A.S. or the equivalent technical program. Details vary, but this is generally how the B.A.S. is structured at most American colleges and universities which offer it.