For thoroughness (and to allow me to find this again later...) here is some additional connection:
The boundaries feature is explored more here: https://bonfirenetworks.org/posts/how_to_boundaries/
My take on the boundaries section of this page is boundaries are how the Fediverse environment affects the user who sets the boundaries. Another take on boundaries would include how I, as a user posting, affects the ecological environment in the sense of computational heat generation in the service of transporting posts beyond my screen and cost of services to do so. This is along the lines of https://www.theguardian.com/environment/series/the-carbon-footprint-of-everything. Me as a user might want to trade my use of email's eco-environmental cost vs using the Fediverse. I want to limit the potential reach, so I know I'm trading to my eco-environmental goals.

It looks like I can use Circles as a way to limit the eco-environmental cost of communicating (nts: see the How-to boundaries link above)