The third scenario is the hybrid Skype for Business, where you have your on-premises connected to the online Skype for Business:

Skype for Business hybrid is a combination of the cloud and on-premises scenarios:

For a complete plan to implement Skype for Business Online in the hybrid mode, visit https://support.office.com/en-us/article/Plan-your-Skype-for-Business-hybrid-deployment-255f0395-1bfa-4360-9cf6-f0d1d82453a4.

To learn more about how to implement Skype for Business in a hybrid deployment, visit https://docs.microsoft.com/en-us/skypeforbusiness/skype-for-business-hybrid-solutions/skype-for-business-hybrid-solutions.