Scope selector
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 of course 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 is 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:
However this won't be sufficient to unbreak the Disqus comment widget, because after you allowed 3rd-party disqus.com, the comment widge 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. Another approach is to create an allow rule for disquscdn.com in the global scope, such that next time you want to allow Disqus on any given site, the Disqus widget will work immediately after you allow disqus.com (notice the global scope is selected below):
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.