What is a shuffle button on an elevator?

Here is the answer directly from Otis:

"The Shuffle button is used only by Otis Technicians when they need to change the placement of the lower/upper deck elevators. It allows them to safely board the lower deck car top from the main lobby. A special code is required from the Elevator management system to allow shuffle operation to be activated."

Keep in mind that the elevators in question are double-decked and serve two floors at once.