Your lovely professors/instructors choose the Required textbooks. Required textbooks contain the basis of the course and will be referred to frequently. Alternate textbooks are variations on the required textbook that deliver the same content but in a different format. For example, your instructor may have selected an eBook and access code as the Required Textbook. However they are also giving you an alternate option of the print version of the book, the eBook and access code as a package. Alternate options provide students options for purchasing the course materials that best fit their learning needs and their budgets. Important note: you only need to buy ONE of either the Required textbook or its alternates.
If you want to be a boss, you can also read the optional textbooks, which are for additional reading on a particular subject.