40 C.F.R. § 1501.12
Agencies shall incorporate material, such as planning studies, analyses, or other relevant information, into environmental documents by reference when the effect will be to cut down on bulk without impeding agency and public review of the action. Agencies shall cite the incorporated material in the document, briefly describe its content, and briefly explain the relevance of the incorporated material to the environmental document. Agencies shall not incorporate material by reference unless it is reasonably available for review, such as on a publicly accessible website, by potentially interested persons throughout the time allowed for comment or public review. Agencies should provide digital references, such as hyperlinks, to the incorporated material or otherwise indicate how the public can access the material for review. Agencies shall not incorporate by reference material based on proprietary data that is not available for review and comment.
40 C.F.R. §1501.12