Every file on your computer has a unique name. The first part of a file name indicates the disk drive that the file is physically stored on. For example, if the file name starts with C:\ then the file is on your C drive. This works fine for fixed volumes, but problems occur when you start using removable volumes such as CDs.
If a file is stored on a CD and that CD is mounted in the F drive, then the file names of the files on that CD will start with F:\. If however, you have two CD drives, F and G, and you move the CD to the G drive, the file names of all the files on the CD will now start with G:\. This is a problem when a program wants to keep track of file names on removable volumes. The problem is caused by the fact that Windows uses drive letters to identify disks. 'Real' operating systems such as that used on the humble Amiga are able to identify volumes rather than just drives. This means that it doesn't matter what physical drive a volume is mounted in.
PMIO works around the limitations of Windows by incorporating a concept called addresses. An address is similar to a file or folder name, but it contains more information and allows files and folders to be stored anywhere without the need for drive letters.
For an advanced discussion of the address system, please see the technical details page.