by Darius Kazemi, Jan 12 2019
Michael Wingfield designed the interface linking the SDS Sigma 7 computer at UCLA with an Interface Message Processor (IMP) in 1969 and implemented TCP/IP for Unix in 1979.
The technical content
The document is a handful of flowcharts extracted from BBN Report No. 1822. Report 1822 was BBN's documentation on how to connect a HOST to an IMP.
Since hooking up a HOST to an IMP was literally Wingfield's job, it makes sense that he'd extract some important summary diagrams and distribute them to the RFC recipients. They probably came in handy and made sense to have as a separate reference document while you were sitting down trying to figure out the physical hookups and data interchange of these machines.
The only available scans of BBN Report No. 1822 I can find online are from its 1976 revision, which is 7 years after this document was written and according to its own foreword varies significantly from the original. I'm going to a bunch of archives this year and I'll try to dig up a 1969 edition.
The Kleinrock History Center hosts a collection of Wingfield's documents including many from the project to hook up UCLA's Sigma 7 HOST to an IMP. A lot of these documents are electrical engineering related rather than computer science, which I think is really neat. For example, these pages contain things like impedance calculations for cables and this page has a diagram of the actual wire-by-wire connections required: