What is the difference between Eldians and The Subjects of Ymir? In season 3 of Attack on Titan, what’s the difference between Eldians and The Subjects of Ymir?
Eldians refer to people living in Eldia, a former nation. They comprise of:


The Subjects of Ymir
The Ackerman Clan
The Azumabito Clan


The Subjects of Ymir are people directly descended from Ymir Fritz. They are also Eldians as they were living in Eldia together with the Ackerman and Azumabito clans. As stated in the wiki, they are the 


   only race capable of becoming Titans.


All Subjects of Ymir were Eldians but not all Eldians were Subjects of Ymir.