These are also called subsystems
There may be a single object - a gatekeeper - that stands as the public representative of the larger group.
And you talk a lot in the book about this concept, this organization, and inter-object communication and how objects communicate a lot in the neighborhoods but then not as much from neighborhood to neighborhood. That's more like, I envisioned it as almost an external API call even though it's probably in the same process or whatever. - James Edward Gray
"A thought process that can yield interesting design insights is to ask what the system will look like if you design it to facilitate testing. Do you need to separate the user interface from the rest of the code so that you can exercise it independently? Do you need to organize each subsystem so that it minimizes the dependencies on other subsystems? Designing for test tends to result in more formalized class interfaces, which is generally beneficial."
One way to ensure separate neighborhoods that act like logical-microservices, but in the same process would be:
"I've worked with microservices in the last year and a half, and I partially agree with the author. My current approach is that of "logical microservices" -- running within the same process/runtime, but with a language-independent API, pure JSON, for example."