What account is used for settings sync?
In Windows 8.1, settings sync always used consumer Microsoft accounts. Enterprise users had the ability to connect a Microsoft account to their Active Directory domain account to gain access to settings sync. In Windows 10, this connected Microsoft account functionality is being replaced with a primary/secondary account framework.
The primary account is defined as the account used to sign in to Windows. This can be a Microsoft account, an Azure Active Directory (Azure AD) account, an on- premises Active Directory account, or a local account. In addition to the primary account, Windows 10 users can add one or more secondary cloud accounts to their device. A secondary account is generally a Microsoft account, an Azure AD account, or some other account such as Gmail or Facebook.
These secondary accounts provide access to additional services such as single sign-on and the Windows Store, but they aren't capable of powering settings sync.
Reference:
https://docs.microsoft.com/en-us/azure/active-directory/devices/enterprise-state-roaming-faqs