class Rails::Application::Configuration

that the application is configured to load that extension.
Note that an extension enabled is not the same as an extension activated or even loaded: it just means

TrustyCms.configuration.enabled_extensions # => [:name, :name, :name, :name]

and for most purposes this is the list you want to refer to.
configuration directives. These are the extensions that will actually be loaded or migrated,
The list of extensions, expanded and in load order, that results from combining all the extension

def enabled_extensions
  @enabled_extensions ||= expanded_extension_list - ignored_extensions
end