From Toon Van Acker:
Virtual volumes have been a part of REALbasic since version 4.5. They allow various kinds of data to be stored in a single file. For example, virtual volumes can be used to combine the data files (sound effects, graphics, music...) of a game into just one file. Using virtual volumes you can easily reduce the clutter of your applications. Virtual Volume Browser helps you to create and manage these virtual volumes. It can browse through your files and handles virtual volumes as if they were folders. Files can be dragged in and out of a window and you can compact virtual volumes to reduce their size.