Just quick reminder about attributes which are synced to AAD. I have been searching once again this information at current project due the problems with sync engines.
Link contains following tables
Table 1: Attributes that are synced from the on-premises Active Directory Domain Services (AD DS) to Windows Azure Active Directory (Windows Azure AD
Table 2: Attributes that are written back to the on-premises AD DS from Windows Azure Active Directory in an Exchange hybrid deployment scenario
And interesting part “How directory synchronization determines what isn’t synced from the on-premises environment to Windows Azure AD”