Meaning of a string

Question. There are several instances but I still don't get how can 
something actually be "in a boundary"? It could be "on a boundary" but 
that equally does not make a lot of sense. I think what is probably 
meant is something like "it's in the cage we built around Facebook and 
Facebook sites". Wouldn't that be better put as "inside the Facebook 
fence" or use the imagery of a pen/corral/stockade/cage instead?

This icon appears in the Firefox address bar when you visit sites, like 
Facebook.com, that are in the Facebook Container boundary.
Comment This is the first paragraph of the second onboarding panel.
Context onboarding2-p1
Resource messages.json
Project Facebook Container

Michael