The Department of Justice regulatory basis for issuing a business review letter is set out in regulations at 28 C.F.R. §50.6, ¶ 2.
This description of the process for requesting a business advisory letter is excerpted below from Chapter III Section H of the Antitrust Division Manual (pdf version) (or HTML version)
This procedure had its origin in what were known as "railroad release" letters, the first of which was issued by the Division in 1939. Under the "railroad release" procedure, the Division would review proposed business conduct and state whether it would forego the initiation of criminal proceedings should the proposed conduct be carried out. This was subsequently expanded to include a merger clearance procedure under which the Division would state its present enforcement intentions with respect to a merger or acquisition. In 1968, these practices were formalized as the Business Review Procedure, and regulations describing the procedure were issued at 28 C.F.R. § 50.6.
The Hart-Scott Rodino Antitrust Improvements Act of 1976 eliminated much of the need for a business review procedure in the merger context. Today, the business review procedure is only used to evaluate potential civil, non-merger, conduct; with the exception of a very limited number of health care mergers, the Division as a matter of policy does not conduct business reviews for proposed mergers.
Generally, each letter sets forth (a) the procedural history of the request; (b) a description of the representations made by the requestor; (c) a statement of the Division's enforcement intentions; and (d) a description of the Division's procedures in making public the information in the business review file. A business review letter must be signed by the Assistant Attorney General, or, in his or her absence, by the Acting Assistant Attorney General.