Quotes About Subdirectories

A union mount is a mount that allows several filesystems to be mounted at one time but appear to be one filesystem. The union mount overlays the filesystems on top of one another so that the resulting filesystem may contain files and subdirectories from any or all of the underlying filesystems.

Limit use of shareware and public domain software to systems without fixed disks. If you do use them on fixed disks, allocate separate subdirectories... Public domain or shareware software should never be placed in the root directory.