Rashi (in Yehoshua, 15:5): It is part of the eastern border of Eretz Cana'an.
2)
Why does the Torah insert the (otherwise superfluous) words "ha'Aretz li'Gevulosehah"?
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Bechoros, 55a: To teach us that the whole of the land is considered one border - and that consequently, if ten lambs are born anywhere in Eretz Yisrael - even if five are born in one tribe and five in another - the owner is obligated to Ma'aser them.
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