An ABSOLUTE must buy. Most AAMC materials are must buys. This is the closest thing to the actual MCAT you will get. Many students choose to take this 1-2 weeks before their exam, as a gauage as to how well they will score. It is not scored, so it's a little less valuable than the official scored one, however there are several scoring paradigms online
Most resources list are 10+, otherwise they would not be effective study tools
0-5 include resources that are either heavily outdated are just cover too much depth (or too little) to be used as a resource for focused studying on the exam. This would include textbooks such as Lehninger’s Biochemistry—filled with great content but is not practical as a study tool.
5-10 are moderately outdated resources (dating back to the old MCAT) that have some relevant concepts, but should only be used if access to newer resources is not possible
10-15 constitute resources that do a good job of covering what you need to know for the exam, but either lack depth or cover beyond the scope of what you need to know in an inefficient manner, and should therefore be supplemented with additional resources. Also, it may describe a resource that dates back to the old exam but still provides relevant content and/or strategies for the new MCAT
16-18 are resources everyone should strive to have as they are strongly recommended because they cover the material, in depth, of what you need to review and/or learn for the MCAT. This includes many of the bulky texts from the “Big Three” (Kaplan, TPR, TBR), Khan Academy videos, which are an essential instructional tool, and valuable practice exams.
19-20 Are must-have’s. This includes all AAMC content, and the practice exams that most closely mimic the actual MCAT. Note that all practice exams are rated 15+ because it is imperative that you continually master how to take the exam in order to do well.