B. Procedural Background
On September 25,1996, appellants wrote to Customs, requesting disclosure of “Customs[’s] harmonized numbers for actual commodity description[s]” of 68 shipments, which appellants identified by their entry numbers. See Letter from Jay Tolentino, NYK Line, Inc., to Audrey Adams, Customs (Sept. 25, 1996), reprinted in App. 15-16. On October 8, 1996, Customs responded by refusing to release the requested HTS numbers, claiming in only one sentence that such information fell within FOIA Exemption 4, which exempts from disclosure “trade secrets and commercial or financial information obtained from a person and privileged or confidential.” 5 U.S.C. § 552(b)(4); see Letter from Adams to Tolentino (Oct. 8, 1996), reprinted in App. 17.
On November 12, 1996, appellants appealed Customs’s initial refusal to the Customs FOIA Appeals Officer. See Letter from R. Frederic Fisher et al., Counsel for Appellants, to FOIA Appeals Officer, Customs (Nov. 12, 1996), reprinted in App. 20-26. They explained that the purpose of their FOIA request is “to compare the commodity declaration made ... to the ocean carrier (as subsequently filed with Customs on the [IVM]) with the commodity declaration made ... directly to Customs [on the Import Declaration].” Id. at 2, reprinted in App. 21. They argued that the requested information is no different in kind than the information released to the public on the IVMs, but it is more likely to be accurate. According to appellants, then, “[i]f the two commodity declarations for the same shipment differ, the only ... reason for according confidential treatment of the commodity declaration to Customs would be concealment of commercial fraud and violations of the Shipping Act.” Id. at 4, reprinted in App. 23. Moreover, appellants noted that local Customs field offices had, in the past, provided them with the HTS numbers for certain shipments. See id. (citing Letter from Alice M. Rig-don, Customs, to Dan Fetters, Hyundai American Shipping Agency (Aug. 14, 1996), reprinted in App. 18-19).
On January 16, 1997, the Appeals Officer affirmed the initial determination that the requested information fell within Exemption 4, explaining that “Customs has long considered information on entry documents to be confidential information, exempt from disclosure.” Letter from Marvin Amernick, Customs, to R. Frederic Fisher et al. 2 (Jan. 16, 1997), reprinted in App. 28.
Appellants subsequently filed suit in District Court, and the parties filed cross motions for summary judgment. Without acting on appellants’ request for oral argument, the court granted summary judgment in favor of Customs. See Memorandum at 10, reprinted in App. 280. The trial court determined that Customs, by the submission of detailed affidavits, had carried its burden and demonstrated that the release of the HTS numbers, when linked by an entry number to a specific shipment of goods, presented a threat to the competitive position of the importers who provide this information. See id. The court also determined that the isolated release of HTS numbers in the past by various Customs field offices did not affect the disposition of the instant case. See id. at 9, reprinted in App. 279 (citing Medina-Hincapie v. Department of State, 700 F.2d 737, 742 n. 20 (D.C.Cir.1983) (holding that unauthorized disclosure of documents does not constitute a waiver of the applicable FOIA exemption)). This appeal followed.
II. Analysis
A. FOIA Exemption J
The Freedom of Information Act requires that federal agencies comply with requests to make their records available to the public, unless the requested records fall within at least one of nine categories of exempt material. See 5 U.S.C. § 552(a), (b). Pursuant to Exemption 4, FOIA exempts from disclosure “trade secrets and commercial or financial information obtained from a person and privileged or confidential.” 5 U.S.C. § 552(b)(4). There is no dispute that the HTS numbers