Prompt Multiple Members, Display All if None is Chosen

I came across this code in one of our existing dimensional reports. This will prompt for members of a certain level, display all of the members of that level if none is chosen, otherwise display only the chosen members. The chosen level is also shown at the left side of the screenshot.

The image below is how the report looks. The columns display the chosen members (with amounts – the columns are suppressed). If the prompt is left bank, all members of the level will be displayed.