This Revit video tutorial briefly shows how to use beam system command in Revit for creating the model of nailers or beams.
The users have to arrange it on work plane. By default, it is connected with your level (finish floor). Remembering it, it becomes possible to apply beam on slope and vertical surface, like uncovered rafter below the roof and purlins on the vertical panel.
To get started with beam system, there should be merely one accessible beam family (structural framing family) loaded to the project.