Wikipedia

Filing under seal is a procedure allowing sensitive or confidential information to be filed with a court without becoming a matter of public record.[1] The court generally must give permission for the material to remain under seal.[2]

Filing confidential documents "under seal" separated from the public records allows litigants to navigate the judicial system without compromising their confidentiality, at least until there is an affirmative decision by consent of the information's owner or by order of the court to publicize it.[2]

When the document is filed under seal, it should have a clear indication for the court clerk to file it separately – most often by stamping words "Filed Under Seal" on the bottom of each page. The person making the filing should also provide instructions to the court clerk that the document needs to be filed "under seal". Courts often have specific requirements for these filings in their Local Rules.[3]

Normally records should not be filed under seal without court permission.[3] However, Federal Rule of Civil Procedure 5.2 allows a person making a redacted filing to also file an unredacted copy under seal.[4]

What Does “Under Seal” Really Mean?
http://www.jameshoyer.com/what-does-under-seal-really-mean/

The federal False Claims Act provides that when a new case is filed, there are certain rules that must be followed to keep the case confidential and to prevent the defendant from finding out that the government is conducting an investigation. The procedure, known as “filing under seal,” requires that a complaint “shall be filed in camera, shall remain under seal for at least 60 days, and shall not be served on the defendant until the court so orders.” 31 U.S.C. § 3730(b)(2). This part of the False Claims Act seems to be a mostly procedural rule for lawyers to follow, but in reality, it puts very important obligations on the relator too.

So what does “under seal” really mean to a relator?

Relator’s Obligations

The False Claims Act’s filing-under-seal requirement exists to give the government time to investigate the alleged fraud, determine whether it is already investigating the alleged fraud, and decide whether it will prosecute the suit itself before the target is tipped off. Various court interpretations make clear that the seal requirement does not impart a requirement of absolute silence about the alleged fraudulent conduct. Such a requirement would place a burden on the Relator that exceeds the language or purpose of the False Claims Act.

As a practical matter, however, prudent practice is to exercise an abundance of caution when discussing any aspect of a False Claims Act case or the underlying allegations with anyone other than the relator’s counsel and the government agents assigned to the case. Most importantly, a Relator should never disclose the actual filing of a qui tam case without speaking with their attorney first. Although a qui tam case is only under seal by statute for the first sixty days, the cases often remain under seal for months or even years while the government investigates, so it is important for a relator to know the exact status of a qui tam case at all times. During the period of time that a case is under seal, a relator must be prepared to exercise a level of discretion that may be difficult to maintain, but is of utmost importance to the success of a case.