Click here and spend ten minutes to read this PDF. (Or, better yet, get the version from the R&A.)
The title of the PDF is “Certain Topics or Proposals Not Addressed in the Proposed New Rules of Golf for 2019” and the topic is precisely that.
For anyone interested in understanding the Rules at a deeper level, documents like these as well as books like “The Principles Behind the Rules of Golf” will serve as a great guide toward achieving that deeper knowledge. Though the Rules of Golf were often relaxed and occasionally simplified in 2019, the core principles, the fundamentals and challenges of playing the game, remain virtually intact as they should.
The R&A has a slightly differently formatted copy of almost the same document here. The differences here? This document covers the final 2019 Rules, while the USGA’s covers the proposed 2019 Rules. That’s why, for example, “no penalty for a double hit” is in the USGA document.