The Eclectic Light Company writes:
Macs and iOS devices have the benefit of not one variety of Rich Text documents, but two: RTF and RTFD. This article explores some of their features and limitations, and considers the problems of working with them alongside one another.
This is a very readable and helpful article that explains the nature of RTFD files and their history going back to the origins with NeXT Corp.
Check It Out: What to Know About RTF vs. RTFD Files on a Mac