To provide greater flexibility to students, your booklist will often include multiple options for your course materials. Required textbooks are chosen by your instructor and contain mandatory readings and content essential to your participation in the course. Alternative textbooks on the other hand, are typically alternate versions of the same required textbook that deliver the same content but in a different format.
For example, your booklist might show an eBook and access code as your required textbook, and list the physical package version of the same textbook as an alternative option. However they are also giving you an alternate option of the physical package of the book, which contains the print version of your textbook and digital access code as a package. If you'd prefer having the physical book rather than an eBook, you can select the print package as an alternate option. 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, as either option is acceptable for your course.
If you want to be a boss, you can also read the optional textbooks, which are for additional reading on a particular subject. Optional textbooks aren't required for your course, these are normally supplementary course materials that are recommended by your instructors.