In a project with multiple buildings and different heights on the plans you can create a custom floor plan and then refine the area belonging to that floor. However, this action is only applicable to the pdf file selected for the floor with a custom floor plan.
If you have multiple drawings for a single floor, you can set an area which is common to all drawings so you do not have to do the same thing several times.
If you want to change your view to a different floor, you will only be able to choose floors from the individual building you're viewing because the delimited floor area is unique to the building.
If you wish to choose between floors in another building, you must be located in that particular building.