davey Where is the boundary?
There are many boundaries.
Apps in the same user profile are expected to cooperate, within limitations imposed by the sandbox infrastructure -- each app runs in its own sandbox but is allowed to communicate with other apps. Apps in different user profiles are somewhat isolated, but only somewhat. If a secondary profile is authorized to, apps in it can use the owner profile's contact database and/or call logs, etc.
The system is complicated -- perhaps too complicated. But any attempt to explain it with one or two simple rules is unlikely to be accurate.