Reply #5: Under Rule 21 of the Senate Standing Rules

[gratuitous]

The Senate conducts its business under a set of Standing Rules and in accordance with Robert's Rules of Order, as well as various other rules that may from time to time be agreed upon by the Senate.

Under Standing Rule 21, any Senator may move that the Senate move into a closed door session. If the motion is seconded, the galleries are cleared of spectators, Senators are required to get to the Senate chamber, and everyone who doesn't have security clearance has to vacate the Senate chamber.

Usually, going into a closed session of the full Senate means they're discussing classified matters requiring security clearance. It's an unusual procedure because these matters are routinely handled by the Senate Select Committee on Intelligence. The reason, of course, is that during a closed session, no other public business can be transacted or considered. It's an enormous inconvenience, and all of the Senate's business halts immediately.

The first thing to be decided during a closed session of the Senate is whether or not to proceed. That requires discussion of the subject matter for which the motion was made, and a majority vote to continue. If the majority votes to conclude the closed session, then the doors are opened back up, and business continues as before.

If the majority votes to stay in closed session, then they stay there until they resolve whatever classified matter is before the body, or until a motion is made, seconded and carried to end the closed session.