The pattern has angered Democrats, who argue that the officials are improperly impeding Congress’s oversight investigation.

Here are several issues raised by the dispute.

What is executive privilege?

Presidents have claimed the power under the Constitution to prevent the other branches of the government from gaining access to certain internal executive branch information, so that, among other things, the president and top White House aides can communicate confidentially with other officials. The idea is to avoid a chilling effect on the candor of the advice the president receives about his constitutional duties.

Normally, a witness who refuses to answer a question at a congressional hearing can be held in contempt of Congress and face criminal prosecution. Congress could also file a lawsuit asking a judge to order the witness to provide the information, raising the further possibility of being imprisoned for contempt of court. But a valid assertion of executive privilege would provide a lawful basis to decline to answer.

How is the privilege activated?

The president asserts that particular information is covered by the privilege. This was the core of the complaint some lawmakers made against Mr. Sessions on Tuesday: Internal executive branch information is not legally shielded from Congress by default; rather, the president chooses whether to invoke it in a particular instance.

Do officials have a legal right to avoid providing potentially privileged information?

No, but Congress rarely demands immediate answers when an objection arises, either. In practice, the executive branch and Congress generally negotiate over access to information that the president believes he has a constitutional right to keep confidential and that lawmakers believe falls within their own constitutional powers to carry out their oversight responsibilities.