Architecture
Regarding the architecture of Skype for Business Online, there are three models:
- Cloud only
- Skype for Business on-premises
- Skype for Business hybrid
Cloud only
The cloud-only model is used as Software-as-a-Service (SaaS). Microsoft has your identity and manages the Skype for Business infrastructure. You can also have some add-ons, such as Microsoft's cloud-hosted Private Branch Exchange (PBX) and the Public Switched Telephone Network (PSTN) calling service:

In cloud-only scenarios, organizations use the Skype for Business service only on the Microsoft infrastructure.
Note
You can learn more at https://docs.microsoft.com/en-us/skypeforbusiness/skype-for-business-online.
Skype for Business on-premises
Your installation and your architecture are maintained on-premises in a datacenter that your organization controls:

In this scenario, customers have Skype for Business deployed in their infrastructure in their on-premises servers. You can learn more about this at https://docs.microsoft.com...