Every rule you create in uMatrix applies to a specific scope. The scope tells uMatrix where a rule should be enforced. For example, you may want to block all network requests to facebook.com everywhere by default (* facebook.com * block), except when visiting web pages from facebook.com (facebook.com facebook.com * allow)
The matrix UI in the popup panel allows you to easily select the scope from which to visualize/create/remove rules, the top-left cell is the scope selector:
It is advised that you create rules which apply only for a specific scope, but of course it's convenient that some rules are created in the global scope (*) such that you do not have to constantly create the same rules for many sites. For most it's convenient to create allow rules in the global scope for domains which use are widespread, and which you trust.
A concrete example. Disqus is a commenting platform which use is quite widespread, so it's a good idea to keep disqus.com blocked everywhere by default (which is the case if you keep uMatrix's default settings). However, there might be times where you want to view the comments or use the Disqus commenting widget on a given site. In such case, you would create allow rules for that site, as seen below (lareviewofbooks.org disqus.com * allow, lareviewofbooks.org disqus.com frame allow):
Sidenote: notice that network requests to referrer.disqus.com, glitter.services.disqus.com, and realtime.services.disqus.com are still blocked despite allowing disqus.com. These block rules are loaded from the HOSTS files. This also demonstrates that narrower rules can't be overriden by broader ones.
However this won't be sufficient to unbreak the Disqus comment widget, because after you allowed 3rd-party disqus.com, the Disqus widget is still broken, and now the matrix shows network requests to disquscdn.com were blocked.
You could just create one more local allow rule for disquscdn.com to enable Disqus on the page (lareviewofbooks.org disquscdn * allow).
Another approach is to create an allow rule for disquscdn.com in the global scope (* disquscdn.com * allow), such that next time you want to allow Disqus on any given site, the Disqus widget will work immediately after you locally allow disqus.com (notice the global scope is selected below):
Rules from a broader scope always propagate to narrower scopes, so rules from the global scope always propagate to all scopes, unless of course a narrower scope override a specific rule. Back to the lareviewofbooks.org scope, we see the global allow rule for disquscdn.com has propagated to the local scope:
If you would rather have Disqus commenting widget not be broken at all on all sites by default, then you can just create the disqus.com rules above in the global scope: * disqus.com * allow, * disqus.com frame allow -- i.e., your choice (I personally wouldn't do this given how widespread is Disqus).
Anyway, this is the purpose of the scope selector, to easily switch back and forth between scopes. When you switch to a given scope, the matrix will reflect the rules in effects for that scope, i.e. what would happen if the network requests had been made in the selected scope, and the rules you create/remove will be created/removed from the selected scope.