Federal Rule of Criminal Procedure 6 says that, unless and until details are revealed by court order as part of an indictment or other proceeding, they must be kept secret. This is to guard against the government releasing derogatory things about people for political purposes.

This creates a unique situation with President Trump. Existing Justice Department guidelines say that a sitting president can’t be indicted — guidelines Trump legal spokesman Rudolph W. Giuliani has said Mueller will abide by. If a president can’t be indicted and the Justice Department can report only the things related to an indictment, that means any wrongdoing by Trump wouldn’t be reported.

Basically, what we learn about Trump may have to come in other criminal filings related to other figures in the case.

“If the only reason that Mueller’s grand Jury can’t file an indictment is the DOJ policy that says no indicting of sitting presidents, then Trump gets the best of both worlds: no indictment and no revealing of the evidence the grand jury saw — even though it may be more than enough to indict any other citizen of the republic," former federal prosecutor Patrick Cotter said.