The Redo tool reinstates all actions canceled by Undo. After reinstating the actions, the current tool continues.
For example, suppose that you place a door and then cancel the placement with Undo. You then choose to place a window. While the Window tool is active, you click Redo. Revit LT reinserts the door and then continues the Window tool.
If the Redo tool is active and you execute an action, the Redo information is lost.
To redo a single action
Revit LT reinstates the action that you had previously canceled with Undo.
To redo multiple actions
Revit LT reinstates all actions up to and including the selected action.