This rule deals with the supported, public framework APIs, for example, those available through the framework global $A.
Why is this rule called “Aura API”? Because the core of the the Lightning Component framework is the open source Aura Framework. And this rule verifies permitted uses of that framework, rather than anything specific to Lightning Components.
Aura.something(); // Use $A instead $A.util.fake(); // fake is not available in $A.util
For details of all of the methods available in the framework, including $A, see the JavaScript API at https://myDomain.lightning.force.com/auradocs/reference.app, where myDomain is the name of your custom Salesforce domain.