The lowest level of National Environmental Policy Act (NEPA) analysis is that given to CATEXs (also called CX, CatEx, CatX, etc.).
Extraordinary circumstances, and ways of screening CATEX assignments to determine whether or not such circumstances exist, are set forth in each agency's NEPA procedures. Based on the regulatory definition of the term "significantly" at 40 CFR 1508.27, agencies typically list as extraordinary circumstances such situations as those where public health may be affected, where wetlands, endangered species, or historic properties may be adversely affected, where there are unique risks or uncertainties about impacts, and where a violation of law or environmental protection procedures may occur. Often an agency will use a "checklist" to tick off possible "extraordinary circumstances."
Some agencies conduct or contract for CATEX screening studies under various names. These studies are done in order to determine whether "extraordinary circumstances" exist that require further review.
Typically, such a study will involve a quick review of the project site and its surroundings, consultation with local authorities and other knowledgeable parties, and a study of pertinent background requirements. It may result in a short written report or simply in completion of a checklist.
A CATEX screening study may provide the context in which the agency addresses other environmental review requirements, such as those under Section 106 of the National Historic Preservation Act (NHPA).
The Section 106 regulations, 36 CFR 800, issued by the Advisory Council on Historic Preservation (ACHP), make a point of requiring agencies to consider whether a categorically excluded project requires review under Section 106.
If a project, activity or program is categorically excluded from NEPA review under an agency's NEPA procedures, the Agency Official shall determine if it still qualifies as an undertaking requiring review under Section 106 pursuant to Sec. 800.3(a). If so, the Agency Official shall proceed with Section 106 review in accordance with the procedures in this subpart. 36 CFR 800.8(b)
This emphasizes that: