Lawyers for Hillary Clinton are appealing a Washington judge’s order requiring her to sit for a deposition in a public-records suit that confronted her use of a private email server as the U.S. secretary of state, setting up a fresh test of the high bar that generally blocks lawyers from getting a chance to question current and former top government officials in person.

Clinton and a former longtime aide, Cheryl Mills, who also was subject to the judge’s order allowing a deposition, on Friday filed their challenge in the U.S. Court of Appeals for the D.C. Circuit. The D.C. Circuit has long grappled with the “extraordinary circumstances,” as described in previous court rulings, that are necessary to compel a high-level official to sit and answer questions in a lawsuit. The standard is often impossible to meet.