Are people only called “subjects” in relation to their monarch, or to their liege lords as well? For instance, the villagers in The Sworn Sword – They are subjects of King Aerys, but are they also subjects of Ser Eustace? of Lord Rowan? of Lord Tyrell?

Good question!

From what I’ve read, the term “subject” can be used to describe the relation of an individual to their immediate liege lord as well as their ultimate sovereign, although it might not have been the most common term used; you might instead use the term “liege-man” or “tenant” to avoid confusion.