Sometimes there are things above the plan cut plane that you want to show up the plan. A ceiling mounted projector and the projector screen are two examples of items that you might want to see on the RCP and the floor plan.
Normally an object hosted on a ceiling will not appear on the plan so you could manually draw in dashed lines for the item. However every time the object moves or changes, you will have to more or edit the lines.
Just adding dashed lines below the object in the Family where the floor would be will not make it appear in the floor plan.
But drawing an invisible line in the Family straight down so that it passes through the View's Cut Plane will trigger it to appear. You can then use the Linework tool to make the lines dashed.
Plan Regions can also be used for this. You will have to create a plan region for each object. So if you have multiple objects of the same Family, the invisible line trick will automatically do make all of them visible.