The Synchronization Engine from the Azure AD Connect processes identities from the AD and the Azure AD. Every directory or repository organizes the data in their method and provides different access methods. The data repositories that are synchronized with Azure AD Connect are called connected data sources or connected directories (CDs). The two supported CDs in Azure AD Connect are the AD and the Azure AD. You can connect any other repository but in a Microsoft, unsupported way. The following diagram shows the basic concept of synchronization:
Now, let's jump back ...