To illustrate, here's an example. If I represent the defense, I will have access to whatever documents are produced to me in discovery and all information filed with the court (with few exceptions related to attorney-client privilege disputes). However, the other party may ask to redact sensitive material from some filings. If the court agrees, I will get an unredacted copy as the attorney for the defendant, but the other party will file a redacted copy that will go into the public record. So if my colleague, who is not working on the case, looked the case up on PACER (the federal court docketing system), he would only see the redacted version. The same goes for fully sealed documents. I will receive a copy as the attorney for the defendant, but if someone unrelated to the case pulled up the docket, it would say "DOCUMENT FILED UNDER SEAL" instead of the actual document.