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The National Environmental Policy Act (NEPA) requires that federal agencies complete an environmental impact analysis of the resources and areas potentially affected by a Proposed Action. However, while all resources must be considered, those resources that will not be affected by the Proposed Action, or impacts that were analyzed in a broader environmental impact statement (i.e., tiering), do not need to be analyzed in detail, or may be incorporated by reference.

43 Federal Register 56003, Section 1508.28 states "Tiering" refers to the coverage of general matters in broader environmental impact statements, with subsequent narrower statements or environmental analyses incorporating by reference the general discussions of the previous environmental impact statement and concentrating solely on the issues specific to the statement subsequently prepared.