Disclosure: I build Schemity, a desktop ERD tool - this post is from our blog and uses it for the examples.
TL;DR: Focused sub-diagrams remove noise, but they also remove context. In Schemity, a context view pays that back: an orange dot marks every entity with relationships outside the view, so a focused diagram never lies about being complete.
A database context view removes context on purpose - and part of what it removes is every relationship that reaches entities outside the view. Schemity gives that information back: any entity with relationships beyond the current view carries a small orange boundary indicator, so the focused diagram never pretends the set you are looking at is closed.
That design came out of a symmetry I ran into while building context views for Schemity. Every focused view does two things at once: it gives you something by removing noise, and it takes something by removing it. Most tools only think about the giving. The good ones pay back what focusing cost you - and the principle underneath that turned out to be more interesting than the feature.
The problem with looking at everything
