Posted to Dropbox, the 22-page document is called the "Visual/Mechanical Inspection Guide". It's dated March 3, 2017 and covers the iPhone 6, 6s, 6s Plus, 7, and 7 Plus. Apple Geniuses refer to it as a "VMI".
"We have one just like that for all of the products," one Apple retail technician told Business Insider. "Used more for the physical inspection and how to determine cost for damage. That's basically half the training for iPhone techs."
Another technician said that they don't refer to it that often unless a customer comes in with an oddball issue.
In the chart below you can see how Apple decides between which issues are Eligible for Warranty Service, Eligible for Out-Of-Warranty Service, and Ineligible for Service.
Things that are Eligible for Warranty Service include Debris under display glass or pixel anomalies, FaceTime camera foam misalignment, or a Single hairline crack to the front glass without point of impact or additional cracks.
Apple will not service devices that have a mismatch between the configuration code and the color, size, or model. It will also not service devices that are disassembled or missing parts, have non-Apple batteries, or catastrophic damage.
Additionally, Apple will not cover cosmetic damage or water damage.
Check out the screenshots below for more details...