As a title, Realm is used exclusively in Andoras by the former vassals of the Kingdom of Andoras, which they carried over from their status as vassals; previously, all sovereign nations on Andoras were dubbed as kingdoms. Although the Realms are all now independent and free from lieges (and therefore free to restore their kingdom titles), the Treaty of Dragonspire that was penned and ratified following the end of the Civil War declared that even though the Kingdom of Andoras was abolished and the members of House Dragomárus were deposed as rulers of Andoras, there was a high desire among most highlords to make sure that such a power does not rise up again. As such, lords are dissuaded from declaring their lands a kingdom -- and although technically illegal under the Treaty, it is more of an advisory than a true law.
There are exceptions to this rule, as well as breakers; Thyllanor is currently in the process of proving its own legitimacy as a reborn kingdom, but claims itself as a titular kingdom in the meantime. Though they need not follow any given process to legitimize their kingdom as no one has the viable means to stop them, those in the government claim that it will make Thyllanor legitimate not only in backing power, but in traditional formality. Edrane, on the other hand, was given permission from the Dragomári when Draegon Dragomárus invaded to keep their formal titles, remaining at a kingdom-level as the Princedom of Edrane.