"Unto" meaning in this definition: "used as a function word to indicate referenc...

Could someone please elaborate what this definition of "unto" means in the Merriam-Webster online dictionary? This is the definition: "used as a function word to indicate reference or concern". This is the usage example: "they became a world unto themselves —Anne T. Fleming".

Note that this is definition number 2, with 1 simply saying that "unto" means the same thing as "to." In the example sentence, replacing "unto" with "to" ("They became a world to themselves") doesn't work quite the same way, so this definition is pretty much just for this usage.

Calling it a "function word" basically just means "yeah, this word doesn't really contribute its own meaning to this phrase, it just helps it work." And by "reference or concern," they mean that the "world unto themselves" paraphrases to "a world concerning themselves, or having to do with themselves (having a scope of reference)."