What DO they mean by it? "Thirty thousand, five hundred fifty-nine square feet, more or less"? One might imagine it like this: "Somewhere between 29,400 and 29,500 we kinda lost count, so we might have ended up a couple square feet too many or too few."
I think that's what they mean. Well, ±.
The "best" part is that even with the ± in there, they still specify the total area to five significant digits. I think I just don't understand real estate.