I thought it was the "center" of your town. Regardless, as you purchase more land to the right side, your dropzone also moves to the right. I am unsure if land "up and down" effects as well.
How about a tab under Settings that lets us designate a building or decoration for this purpose? I'm sure someone has mentioned it in the past, so apologies if I'm being redundant.
Edit: I see 4Junk mentioned something similar.
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