Skype for Business 2015 and Skype for Business Online Licenses.
Please ask if you have further question. I'm happy helping you in understanding it.
https://technet.microsoft.com/en-us/library/skype-for-business-online-service-description.aspx
Please ask if you have further question. I'm happy helping you in understanding it.
https://technet.microsoft.com/en-us/library/skype-for-business-online-service-description.aspx