How is the main frontend menu generated? (before Magento 2.1.0-rc1)

Development
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The behavior has been changed from Magento 2.1.0-rc1:

https://github.com/magento/magento2/commit/078af98a52912fbae0142577ae4da422bb6705dd

How is the main frontend menu generated? (from Magento 2.1.0-rc1)