When you go back to your room schedule, you'll see something like this. You can see the area is replaced with "Not Placed".
This is fine, but what if you have a few "Not Placed" rooms and they're all over the schedule. It may be tough to find them. Wouldn't it be nice to see them all together, or to hide them, or have some control?
You can! Check out the Options dialog box!
Here's an example of Isolate. The schedule will "only" show rooms that are not placed.
(You could also do the opposite and "hide" all the "not placed" rooms in the schedule...)
Later on if you decide to place the room somewhere else in the model, you can select it from the Options bar during placement. All the "not placed" rooms are fair game along with "new" rooms (which is the default). If you no longer want the room, you have to delete it from the schedule.