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Simple.. the I-140 is part of the Green Card process.. First (in most cases) you have to prove that you cant find a qualified US resident for the position. This is done via the Labor Certification.
Once you have the LC for the person, you need to file a I-140, an Immigrant Worker Petition, based on that LC. Once this is done, the person can apply for I-485 (adjustment of status) and get the Green Card.
HTH