Apple's Visual/Mechanical Inspection (VMI) guide details how Apple Care centres handle in-warranty and out-of-warranty repairs. The leak of this guide is not exactly path-breaking but definitely offers an insight into how Apple honors warranty and decides the service eligibility of a device — in this case, the iPhone.
https://www.notebookcheck.net/Leaked-VMI-document-offers-an-insight-into-Apple-s-warranty-repair-rules-for-the-iPhone.246308.0.html